"A proper Mexican burrito is made with a paper-thin, crepe-like flour tortilla," explains Thomasina Miers, her voice getting ever-so-slightly louder and the words spilling out increasingly quickly. "You can hold them up to the light and see through them and they're mottled with lovely charred bits. They're enriched with lard, they're stretchy and they're incredibly delicious. They have refried beans and meat in them and that's pretty much it. They're mouth-watering." As co-founder of the 20-restaurant-strong Wahaca empire, and its younger, Mexican-American sibling DF Mexico, she knows what she's talking about.

Thomasina Miers

Photograph by David Harrison

"They're not like these things we have, with rice," she continues, this time with considerably less enthusiasm. "All these burrito places have opened and there's an idea that this is what Mexican food is, but it's still not really Mexican in my book."

Her 'book' is a pretty authoritative one – having fallen in love with Mexico's food on her gap year, Miers spent 13 years thinking about it before, freshly armed with the 2005 Masterchef title, she set up Wahaca in 2007. At that time, Mexican cuisine was synonymous with too much cheese and sour cream, and heavily influenced by poorly executed Tex-Mex.

London's got all these burrito places, but you can count on one hand the number of people doing innovative Mexican food

That was nine years ago. Since then, Mexican cuisine has been championed by star chefs including Noma's René Redzepi, Jason Atherton, James Lowe of Lyle's, and Isaac McHale of The Clove Club, and it's been touted as one of the biggest food trends of 2016. But just how much have things moved on from these negative preconceptions?

The answer is simple, says Miers: not much. "It's interesting to me, because we've got all these burrito places, but you can count on one hand the number of places doing innovative Mexican food," she explains.

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Given that London is full-to-bursting with international food, most of it at an incredibly high standard, this is surprising. But it's something that will always be a problem when a cuisine relies so heavily on ingredients that come from thousands of miles away: "The quality issue is still difficult," Miers laments. "We ran a series of supper clubs with some leading Mexican chefs at Wahaca last year, and each time they brought a suitcase of ingredients such as fresh herbs and corn over here with them. Those challenges are still very real."

This is also why, Miers believes, we're yet to see much high-end Mexican cooking in London. "In the early days, I had a dream of opening a River Café equivalent for Mexican food," she admits. "The trouble is, when you go to that level, it's so much more about every single ingredient. And you can't fake that."

That said, Miers was never about complicated, technical cooking. "I'm definitely more on the feminine, nurturing side of food," she says. "I love the street-food element of Wahaca; I love that it brings everyone together. It's amazing to eat in [higher-end] restaurants, but for me, food is about nurturing and sitting with family and friends and feeding people and making them feel good. That's the thing I love about Mexico: there's this love of breaking bread together – it was that sense of community that was the inspiration.

"If you go to a market for breakfast, you might get fruit juice from one stall, but you'll be sitting at a stall that makes barbecoa. Someone else will bring you tortillas and you'll get your avocados and spring onions from someone else and then the hot chocolate woman will come along, or someone else will ask you if you want some beers. And everyone works together to give you this amazing experience."

As a result, Wahaca focuses on Mexico's vibrant, bustling markets. Since its inception, Miers has become an authority on food – and not just Mexican. She's appeared on TV shows; writes a regular food column for the Guardian; organised last year's Day of the Dead festival at Wahaca; and has written several cookbooks, including 2014's Chilli Notes: Recipes to Warm the Heart (Not Burn the Tongue), which has helped her become synonymous with the fiery red peppers. It's funny, really, considering she stumbled across them almost by accident: "I turned up in Mexico on my gap year – I was quite wild, I drank a lot of tequila. I remember eating ceviche and really hot salsa, and trying to sweat it out. It all started from there."

For me, food is about nurturing – feeding family and friends and making them feel good

Despite her hectic schedule, she still visits Mexico every year, as do her restaurants' chefs, and she brings new discoveries back with her each time. "I feel that a lot of the dishes at Wahaca you could get as a modern adaptation of Mexican food in Mexico City," she says. "It's just made a little more chichi. Maybe the herbs would be a bit more interesting and a bit less British. I use chervil and tarragon a lot because there are these wonderful fresh herbs in Mexico with anise notes, which you can't get here."

Thomasina Miers

Photograph by David Harrison

It's this willingness to innovate which enables Wahaca to stay as authentic as possible. "Real sticklers sometimes send me emails asking why we've got feta if we're Mexican. We use feta because shipping fresh curd cheese from Mexico is completely insane, and feta is the closest thing we can get to the fresh curd cheese that you get there."

All of Wahaca's tortillas are made by one woman and her "little tortilla machine", while queso fresco comes from Gringa Dairy, run by American-born, Peckham-based Kristen Schnepp; all the meat is British, and all the fish is MSC-certified. "I'd love to use octopus or tuna, which you see a lot in Mexico, but you can't get MSC certification on them, so we don't use them," she explains.

"We have a super soft sobrasada on the menu at Wahaca, which is like the spreading chorizo you get in the markets in Mexico. It's just delicious, and it's made by Trealy Farm in Wales. That, for me, is a fun thing." Even the beer, Lupulo Pale Ale, is brewed for DF Mexico by Brixton Brewery.

Wahaca and DF Mexico's dishes – in pictures

Miers isn't daunted by the challenges of remaining as true to Mexican food as possible – a passion shared by her team. "We really wanted to use the authentic process to make the proper corn tortillas, which is called nixtamalization. You actually cook the corn in an alkaline solution, grind it, make the dough and then make the tortillas."

As a result, her development chef, Chris Buckley, spent a week in the Nordic Food Lab – René Redzepi and Claus Meyer's institution devoted to investigating food diversity – with Mexican chefs, trying to replicate the tortillas using European grains such as buckwheat, rye, black beans and mung beans. And this, ultimately, is exactly what Wahaca – and Miers – is all about: "I love Wahaca and I love what we do," she says, "but it's when Mexicans come in and also love what we do that I take the most notice."

"There's always going to be this thing of, 'Who is this British girl who's taken our food? Is she taking it seriously or is she just trying to make money?' They've all gone back to Mexico thinking I'm doing good stuff," she says, with a touch of pride. "They know I care and it's not just a business." And this passion, attention to detail and drive for authenticity is why Miers succeeds where others have failed. Just thank the tequila and chillies.

wahaca.co.ukdfmexico.co.uk