Last night I became one of those people who, like the worst TV spoilers, tells you everything about your meal before you've taken a bite out of it. As we tucked into hake, anchovies, cheese – even the apple sauce – I loudly informed my fellow diners at Eneko at One Aldwych where their food had come from and who it was produced by.

"I've been to the vineyards," I announced, when the waiter presented the vintage txakoli from Gorka Izagirre – recalling as I did so how the undulations, heavy downpours and gusty sea breezes of the Biscay shaped the grapes, which became the wine we were drinking. "I met him when I was in the Basque Country," I beamed at the waiter as he explained how the grapes were cultivated and the wine created by the chef's cousin, Bertol Izagirre, and his team.

I carried on like this through four courses, deaf and blind to all but the food and my – in hindsight, slightly irritating – wealth of insight. The thing is, in May I'd visited Eneko Atxa's original restaurant, Azurmendi, and explored the vegetable plot, txakoli vineyards and the dairy behind it. My excitement, four months later, of being in a plush Covent Garden dining room, enjoying the same uniquely flavourful Basque produce, was hard to contain.

The experience was exceptional. I'd gone because, while not the first Basque-based chef to set up shop in London, Atxa was certainly the most famous to announce the opening, back in January. His London debut crested a wave of pop-up restaurants, permanent restaurants and recipe books from or inspired by the mountainous, Michelin-starred landscape surrounding the icy waters of the Bay of Biscay, Spain.

From to Amesta with Arzak Instruction in Belgravia to Xiringuito in Margate, via Lurra in Marylebone and Shoreditch's Sagardi bar and grill; with each new opening came a new take on the cuisine, not to mention a slew of unfamiliar words full of Xs. Having for decades served to deter visitors, the words 'Basque movement' these days are more likely to refer to this food trend than the notorious ETA terrorist group, which blighted tourism to the region for years.

diners tuck into Basque food at

Photography by Joseph Fox

As Monika Linton, founder of the long-established Spanish deli Brindisa points out: "While we've never had borders in the range of products we source – we want to reflect as many regions of Spain as possible – the political separatist group ETA had a big effect on foreign visitors to the Basque Country for decades." Indeed, so little did the wider public know that when City worker Nemanja Borjanovic and his partner Melody Adams visited in 2010 they were astonished.

"It was so good: the pintxos bars, the fresh fish, the culture… We were just normal people with City jobs at the time, but we kept saying someone in London should do a Basque restaurant – then joking that maybe we should do it."

A year later, having visited a second time and with no such restaurant forthcoming in London, they decided to bring the mountain to Mohammed themselves. They called it Donostia – the Basque name for the city of San Sebastián – found a Basque chef and suppliers and served pintxos, the likes of which you won't find in your average tapas joint: crispy fried cod cheeks with black squid ink aioli, or white anchovies with marinated peppers. "It was," Borjanovic says of its nerve-wracking debut, "an instant success."

"We couldn't believe it. Eventually Adams quit her job, but I was still working in the City in the day and moonlighting as a waiter at night." When, eventually, Donostia became so big Borjanovic's colleagues were paying it a visit, he knew something would have to give. "One night someone really senior in my firm showed up – and I ended up ducking behind the counter so he wouldn't see me!" he laughs. He quit his City job not long after, and set about managing not just Donostia, but the opening of his second Basque restaurant, Lurra: a less rustic establishment inspired by the region's unique breed of beef.

They'd returned to the Basque Country, with no immediate plans to open another restaurant, but as they ventured outside San Sebastián and along the coast a new opportunity loomed. They drank in old cider houses, sampled strange cheese and ate turbot at Elkano: an old family grill turned Michelin-starred restaurant in Getaria where they serve the whole fish grilled on charcoal and basted in local txakoli wine.

"We never say we are chefs," insists owner Aitor Arregui when we visit and he feeds us what feels like the entire Biscay, "we say we are sons of fishermen. We work with the fish we receive daily, because each is different and needs treating differently according to the species, seasons and even the time of day."

Elkano's turbot would come to play a co-starring role at Lurra – but as the name (it means 'land' in Basque) suggests, it's not their headline act. "One night we were staying in this guy's apartment and he asked us if we liked steak. He then took us to a restaurant where they've served nothing but steak, tortilla and tomatoes since 1980." Day in, day out, a queue of people would form outside this small, run-down bar – and it could only be explained by one thing. "This beef, when we tried it?" Borjanovic whistles in remembrance. "I mean, fuck me, it was different. It was like no beef we'd ever tried before – ever. It was something else."

He asked the owner, who informed him that the cows were over 14 years old: an unheard of age in this country where most cows are slaughtered before they reach three years old. At face value it did not seem that selling steaks from old cows here would be feasible: our perception, he feared, would be that it was stringy and tough.

When he came home, Borjanovic introduced the old-cow steak (known as txuleton) to Donostia as a side portion, and was overwhelmed by the response. "People went crazy. They were pre-booking it– we were actually selling out of that dish in advance – and we were supplying some of London's best restaurants with it, too." Taste it and you'll understand: the longer life results in an intense flavour and rich fat marbling – and inspires a deep and unfamiliar connectivity with the animal and the land from whence it came.

The beef in the basque country was really different. it was like no beef we'd ever tried before

"It's ingredient-led", says Borjanovic of Basque cuisine: a definition I would argue holds the key to its surging popularity in a city where such words are rendered meaningless by marketing materials. "Artisan, hand-picked, small producers, organic – these are buzz words in London, but they aren't in the Basque Country. That is just what they do."

They aren't selling a story – indeed, there's no story to sell, Borjanovic continues, for until recently, old cow was only found in the Basque Country: the grill-charred, succulent steaks a staple of the old cider houses which pepper the rolling countryside. Ingredients-led is not a 'thing', it's just what happens: the people eat locally and seasonally, from the market, because to do otherwise is as inconceivable to them as wolfing down a sandwich on the Tube.

To my amusement, the whole concept of a sandwich is met with genuine shock by winemaker Izagirre and another of our hosts in the Basque Country, Maite Diez. The Basque export manager for Brindisa, she is our guide to their addictive, pickled guindilla peppers – a protected-designation product – and the inimitable cigar-shaped biscuits, cigarrillos.

"To have a sandwich here? No. When we are eating, we are sitting down with people and enjoying our produce." Even the most trivial meeting demands at least some pintxos and a glass of txakoli, Maite continues. We see this at coffee time at Casa Eczia, when the third-generation producer of cigarrillos Javier Ecezia plies us with five different types of cake and biscuit; and we see it again when, barely an hour later, our plea for a light lunch "because we're eating a six-course meal at Azurmendi this evening" is met, courtesy of the pepper farmers, with a large cold salad of scarlet-hued octopus tossed with cracked pepper and olive oil, followed by a thick stew of garden vegetables and an ancient variety of grain – and the half sweet, half hot pickled peppers on the side.

"It is a genuine food culture. I know a lot of countries say this about themselves, but when you go there you really see it. It is different," Borjanovic warns us before our trip. The obsession is tangible: "we always say that when we are eating lunch we are thinking about dinner," Izagirre explains, when I ask him about it in Azurmendi's sun-dappled vineyards later, a saying oft repeated, we discover, by his proudly food-centric countrymen.

"It's why we have our gastronomic societies" – collectives of local men and, increasingly, women who gather regularly to cook and eat. Sometimes it's Izagirre cooking, sometimes the local cheesemonger or fisherman – sometimes Eneko Atxa himself, cooking a simple meal for his fellow members. "All the Michelin-starred chefs in the Basque Country are members of these societies in their town – membership is passed down from father to son or daughter," says Borjanovic. "So of an afternoon you'll easily find a famous and pioneering chef like Juan Mari Arzak sitting around with a couple of local guys cooking and chatting about food."

local, seasonal eating isn't a 'thing', it's just what happens

It's all of a piece. The same passion that fuels Arzak's restaurants here (he and his daughter Elena are behind Ametsa with Arzak Instruction) and in San Sebastián pervades the raucous cider houses and the bright, clattering pintxos bars. Arzak's tacos de vacuno con tomatillo, the steak and chimichurri pintxos and the slabs of txuleton slammed down in the cider houses are all branches of the same tree.

Their ingredients are sourced on the same principles – quality, local, tradition and care – and treated accordingly: the service at the cider houses might be brusque, but the respect of the chefs for the meat they cook is as great as you'll find in any Michelin-starred kitchens: perhaps more so, given the historic role they've played in the development of Basque cuisine.

Not for nothing did Borjanovic and Adams go to one of the country's oldest and most renowned cider houses to find not just their cider, but their formidably talented head chef, Daniel Seifi. Perched high in the forested hills frowning over San Sebastian, the seemingly timeless stone-flagged buildings of Petritegi are in fact more than 500 years old. Outside, the apples ripen and fall; inside, long wooden tables and burly oak barrels serve the needs of everyone from raucous family gatherings to bands of bantering young men and women: the cider, I should point out, is bottomless, poured at a height from the big barrels adjacent to the dining room at regular intervals; and just as this recipe, using only windfall apples from their orchards, has been unchanged for generations, so the non-negotiable set menu carries a significance that dates back hundreds and hundreds of years.

Cod omelette, hake, steak, and idiazabal cheese with quince are set down unquestioningly between the txotx – the cider toast called at ten-minute intervals. Each course is the result of the cider house's historic function as a resthouse and trading post for beef farmers and fisherman traversing the country peddling their wares. Over time, the act of trading food at a house of booze became a knees-up in which "they drank the cider and cooked the fish and meat they'd brought", and the inclusion, Borjanovic continues, of a large grill in the house became as much of an institution there as the txotk: the rallying cry in which, in the name of journalistic integrity, we enthusiastically join.

Mass-market cider this is not. Brewed solely from specific Basque breeds of apple, without additives, this cider is sharp, unfiltered and potent. It's served little and often, and Borjanovic has lost more than one chef to the stuff while on staff jollies out there. It is quite flat – hence the practice of firing it out of the barrels at a height to give it fizz, and downing it while the fizz is still there.

eneko atxa serves cheese made by his mother's cousin’s son

The serve at his restaurants is more civilised: there's waiters, and bottles, and people are seated rather than standing up and swaying, glass in hand. It's no Petritegi, but the sight of a smartly dressed waiter aiming at a glass from a great height is worth witnessing. Meanwhile, for the more rustic food and banterous atmosphere of the native cider house (if not the barrels) head to Shoreditch for the newly opened London outpost of cider-house style restaurant chain, Sagardi.

"I'm not surprised that it's taking off. Londoners love their fresh flavours, fantastic seafood and bold plates,' says José Pizarro: founder of three award-winning restaurants and something of a pioneer of Spanish food in London. Though not himself from the Basque Country, his latest recipe book is lovingly dedicated to its chefs and dishes and their significance to Spanish cuisine.

These aren't as 'new' here as we think: "Places such as Barrafina and Iberica will always have had dishes typical of the region, as will every good Spanish restaurant and deli in London," he continues – it's just we haven't noticed it before. Indeed, the preternaturally successful chef of Barrafina, Nieves Barragan Mohacho, is Basque. For my part, I thought I'd never gone near anything Basque before Brindisa's Linton – who, along with running several Basque suppers in her London restaurants this year, has just released a book of Brindisa recipes – informed me it was home to both my beloved cigar-shaped biscuits, and my favourite brand of posh canned fish, Ortiz.

A Basque pintxos

Photograph by Joseph Fox

Once again we're back to the simple merits of the Basque's raw materials: from the soft, mild butter necessary for Ecezia's delicate cigarillos to the Bay of Biscay and its icy wealth of tuna, sardines and anchovies. It's caught by rod and line according to the bay's strict fishing seasons, and the 120-year-old company still fillets, salts and cans the fish almost entirely manually.

Back at One Aldwych, it's time for the cheese course. Though graced with the 'molecular' techniques and dazzling flourishes for which Atxa is famous, there are several courses showing the clear 'line' from his and other Michelin chefs' strikingly modern take on Basque food to its foundations in good ingredients and good company; and it's a line that is most evident in Atxa's cheese.

His mother's cousin's son makes it, on Erotik: a small farm and diary perched high above the Atxas' home town. Azurmendi and Eneko at One Aldwych are the biggest clients; the rest are local people: old women who totter wheezingly up the hill, wicker baskets in hand (yes, really) and young excitable kids sprinting past them, on a mission for cheese. He may be barely 25 years old, but this cheesemaker is well known and respected among the villagers for his dedication to the area's cheesemaking traditions.

"Usually his cheese is served as an elaboration at Azurmendi," Izagirre explains, above the bleat of goats when he takes us to visit: "liquid cheese balls, or a cheese cream for desert..." Yet at Eneko at One Aldwych it is the thing itself: sliced and served, as it comes, with quince jelly. Before I tuck in, I leave my diners with one last morsel of insight. "You know the name of the dairy, Erotik? It comes the Basque word meaning, 'from the roots.'"