Thomasina Miers is late and flustered. She leaps off her bike at Borough Market, South London, mop of curly brown hair bobbing furiously and long legs trailing from a denim miniskirt. It’s an appropriate place to discuss her scrumptious new book Cook, which is mindful of the seasons and the ethics of buying ingredients.

Here we can wander around the stalls until she’s let me in on her secrets: namely, how she became the food world’s hottest property (after winning the BBC’s TV cookery competiton Masterchef last year), tapped into prime food concerns of the day and mastered shopping in local markets. But she’s got to get to Hampshire in the afternoon to be filmed for a programme on foraging. Then it’s back to London to cook supper for the food guru Prue Leith.

She looks as if she might do a runner.

Blackberries save the day. She spots them heaped on a table behind us. Face brightening, nightmare schedule forgotten, she reaches over for a squeeze: “Aren’t they glorious?” And she’s off. Telling me how to make blackberry granita (“Whizz them with sugar and lemon zest in a processor, then freeze”); dashing over to inspect flat-topped Italian peaches (“Best soaked in sweet wine with a few cherries”); and greeting stallholders, most of whom know her, shouting out “Tommi” and slapping her on the back.

Miers, 30, is a hit. Not just with market vendors; she gets on well with everyone. Food celebrities have fallen into her lap, contributing recipes to her first book Soup Kitchen, which she co-edited last year. Spend any time in her company and familiar first names are tossed into conversation. Clarissa (Dickson-Wright, the chef) is credited for encouraging her to follow her passion for food by studying at the Ballymaloe Cookery School in Ireland; Rose (Gray, of the River Café) inspired her with the seasons; Hugh (Fearnley-Whittingstall) wrote the foreword to Soup Kitchen. The latest book offers her own recipes. Pages are peppered with shots of Miers cycling through markets with loaves of rustic bread and strings of garlic. It’s a vision of how right-on your life could be if you could kick your Tesco habit.

Is that the idea? “To encourage people to visit markets, yes,” she says. “It sounds naff but I’ve always been worried about the planet. I was a very serious child; I thought I could change the world. Now I’m realistic. I’ve chosen food as my battleground. We’re losing the art of cooking and destroying the planet in the process. We’ve no fish left in the sea and we’ve lost 97 per cent of fruit and veg varieties in this country.” She takes the campaigning side of her work seriously, pushing for markets to be kept open nationwide, working closely with the New Economics Foundation and the Sustainable Development Commission.

As we pass specialist goat cheeses and homemade pickles, I mention that markets are great as long as you can afford them. “Not true,” protests Miers. “Traditionally markets have been the way to give poor people access to fresh fruit and vegetables. If you want to give people below the poverty line cheap, fresh food, you need to give them access to the producers. It’s not practical to say to a housewife with five kids, never go to a supermarket. Some things, like olive oil, will be cheaper. But fruit and vegetables won’t, and you won’t be able to find cheap cuts of meat, such as brisket.”

Point out that not all of us know our giblets from our chitterlings, or how many grams of each we want, and she’s unfazed. “At markets you’ve got to ask. Be curious. Vendors are keen to help people. If you’re trying lamb neck or skirt steak for the first time, tell the butcher how many people you’re cooking for and ask how much you’ll need and how to cook it.”

Blame Miers’s upbringing for this responsible approach to food. Both parents loved cooking but could afford to buy only cheap cuts of meat. “We lived in a rundown house in Acton, West London. I got interested in food by thinking up ways to spice up the same old stuff. My parents didn’t believe in eating out. As kids, if we pestered them for a McDonald’s, Dad would head to a fishmonger to get some prawns; Mum would make some mayonnaise and buy a fresh baguette and a bottle of wine, and they’d whip up a meal, saying this is fast food. Then we’d be made to work out how much it cost so we realised what good value it was.”

If it wasn’t an affluent childhood, it was genteel, with bohemian touches. Her grandmother was a model who married Pat Fairfield, the Formula One driver. Her mother did a stint as a model, too, as did Miers, at the age of 14. But her passion for food won out. As it did over a natural gift for numbers, which yielded a bursary to St Paul’s Girls School, in London, followed by masterclasses at the Royal Society of Mathematics and expectations from her father that she would be an accountant. Being a maths whizz, surprisingly, has had little impact on her kitchen skills: “I’m not a precise cook but I like being able to do quick sums when I’m shopping.”

She has, however, made some financially smart moves. Her investment banker boyfriend Mark supports her at the moment — no bad thing considering ambitious plans to open a Mexican restaurant next year. “I’m in the middle of an ethical nightmare regarding what to put on the menu,” she says. “There are fish dishes I’d love to include, such as tuna sashimi, but my policy on sustainable fish rules it out.”

When I leave Miers she is wedged between punnets of blood-red berries having her photograph taken. Again she looks uncomfortable and keen to escape. Perhaps she’s remembering her hectic day head, or working out how many food miles there are between here and Mexico.

Cook (published by HarperCollins, October 2, £16.99) is available from Times Books First at £15.20. Call 0870 1608080 or visit www.timesonline.co.uk/booksfirstbuy

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