Spice up your life: Thomasina Miers reveals how to tackle tacos - and more - at home

Thomasina Miers brought authentic Mexican food to the masses. Matilda Battersby meets her.

Thomasina Miers arrives for our interview at Soho Wahaca, says hello and then puts a hand to her face and groans, explaining in her trademark clipped English accent that she'd forgotten we were sending a photographer to take her portrait. Gesturing embarrassedly at her (perfectly elegant) jeans and grey jumper, she dashes off to the loo to hastily apply some mascara.

Running six restaurants, having a one-year-old daughter, writing cookery books, being on telly and establishing a new pop-up restaurant ("It's called Experiment because it is one") on London's South Bank is evidently as knackering as it sounds.

The 2005 MasterChef winner, known to her friends as "Tommi", appears to be running on empty, sipping gratefully at green tea and stifling yawns, while still (somehow) managing to convey the enthusiasm for Mexican cooking that has led her to establish her empire and become a major foodie name.

"After years of feeling bored with my life I felt suddenly happy and terrified," she says, describing the crazy early days opening the first Wahaca, which she and business partner Mark Selby established five years ago.

Wahaca has since become a fresh and vibrant part of the London eating landscape, notching up handfuls of awards and changing the perception of Mexican food from refried cheesy stodge to zesty, colourful melt-in-the-mouth quesadillas, ceviches, tostadas, moles and other chilli-infused delights. "Setting up a business like this is all-consuming, like having a child. We really didn't know what we were doing at first. I believe the only reason we survived was because our team was so passionate. We sweated blood and tears in those first couple of years," she says.

Before finding out that she could cook for a living ("which took me ages to work out") Miers tried a rather eclectic selection of career paths: VAT analysis, modeling, financial journalism, advertising, marketing, digital strategy, secretarial work. "I basically couldn't stick at anything," she says. "I was totally unmotivated and actually thought there was something inherently wrong in my nature, that I was a flawed person; damaged goods."

At 18 in Mexico City during her gap year Miers had made a discovery: "Friends threw a party for 50 people and the table was filled with dishes – like an Ottolenghi window display. But I didn't recognise any of the ingredients. It was intriguing. The flavours were different, incredible, complex. What was this food? It was Mexican!"

Returning to England for university, Miers put this newfound enthusiasm aside. "I studied languages and economics at Edinburgh, which I loved. I'd applied to Oxford but totally screwed up my interview by smoking too much pot," she says, laughing. "It's a shame, really. I saved the rebellious teenage thing for 17, so when the Oxford interviewers asked me about my French and Spanish studies I couldn't remember the names of any of the books I'd read. Not my finest hour." A degree and a litany of failed career attempts later, Miers "had an epiphany" aged 26 and spent six months at Ballymaloe Cookery School in Ireland. She loved it so much she stayed on in Ireland working at a cheese farm and baking bread to sell at farmers' markets.

"But thoughts about Mexican food kept nagging me. Was the food really that great? Or was it just a dream? I needed to go back and find out." So she took a job running a cocktail bar in Mexico City and used every opportunity she got to travel the country eating different foods.

"It was quite a weird time, really," she says. "I was 29 and quite a few friends of mine were getting married, a few of them were having babies. Here I was in this mad country, for what? I didn't know. It was nothing more than some kind of weird hunch."

In 2004, heavily in debt to her parents, Miers returned to London to sleep on friends' sofas. "I'm nearly 30… I really need to get on with my life," she says she was thinking. She decided to open a restaurant. But with zero experience, no contacts and no money, she had no idea where to start.

"I saw an advertisement for MasterChef and (rather bashfully) entered it," she says. "I didn't tell anyone because it was a bit embarrassing. I went to the audition rather cockily, taking it as a bit of a joke. But I became absolutely petrified as these big television cameras turned on me and I had 20 minutes to make the perfect mashed potato."

The first of only two female MasterChef champions, Miers can also be safely described as the programme's most successful alumnus. But it didn't come easily.

"After MasterChef I had four months of examining my tummy button thinking: 'I've won a television competition. That's great, but what does it actually mean?'" Not a lot in material terms, apparently. So Miers contacted Skye Gyngell, head chef at the Petersham Nurseries Café, and began cheffing for her.

The hunch paid off once again and Miers soon met Selby, who was looking to invest in a restaurant. A trip to Mexico, where they ate an average of five lunches a day (you'd never know it from Miers' slender frame) and Selby was sold. Wahaca now employs around 400 staff and with a range of Wahaca chilli sauces hitting supermarket shelves and Experiment opening at the end of the month, it looks set to keep growing.

"It was hard at first to change Mexican food's bad name. My friends thought it was cheap, greasy, cheese-filled and washed down with nasty tequila. We did surveys of people on the street and they were, like: 'Mexican food? Greasy. Quite fun if you're pissed enough.' But I had complete faith."

With gap-toothed glamour to rival that of Vanessa Paradis, it's unsurprising television studios came calling. Miers co-presented two Channel 4 series, Wild Gourmets and A Cook's Tour Of Spain, recently popping up on Channel 5's Mexican Food Made Simple.

Her next mission is to get people cooking Mexican food themselves with the release of her new cookery book, Wahaca: Mexican Food At Home: "Some Mexican food, such as the very complex moles [sauces], do take a long time to cook, but other things are incredibly easy. It's all about layering simple flavours: citrus, salsas, chopped onion, shredded lettuce, fresh herbs, cheese, cured fish. Once you unthread the building blocks of the food you realise how doable it all is. With the slow-cooked meats, they just pack in herbs and spices (cloves, allspice, cinnamon), onions, carrots and celery with a chilli or two and just put cheap cuts on to cook slowly over hours. There is nothing daunting."

Despite the slog getting to this point, and her evident exhaustion, Miers is still exhilarated by the whole thing. She's already fundraising for a street charity in Mexico City and is eyeing up setting up a Wahaca Foundation to help get young people in this country into employment.

"If I were Prime Minister I'd probably bring National Service back in and make every young person work in a kitchen for a year," she says. Prime Minister? I wouldn't put it past her.