Can a menu be sourced solely from London produce?

Pylons loom over marshland, breaking up bleak horizontals of roads and ditches. This is the legendary A13 from London Essex, an industrial hinterland soon to be transformed by John Prescott's Thames Gateway development. Frankly, its brownfield landscapes and docks don't suggest food production. But at Canvey Island, the low-slung Essex coastal settlement, I arrive to do a day's fishing in the Thames Estuary, along with chef Oliver Rowe.

It is Rowe's year. Previously an actor, he discovered food and, upon graduating from London's Moro, set up Konstam in 2004. Rowe has just opened another restaurant in King's Cross, to be called Konstam at the Prince Albert. Its unique proposition is that all the food will be sourced within the bounds of the London Underground tube network.

It's an odd location for such an experiment, King's Cross is infamous for crack and prostitution: indeed, Konstam is located next door to a "massage" parlour. "A bit of gentrification is good," he says. "But I hope it doesn't lose its edge." The idea arose from Rowe's allotment in Barnet when he idly mentioned to a friend that he wanted to offer food produced in Greater London. Now, along with the new restaurant is a television show. Indeed, it shows that the metropolis is surprisingly bountiful, with flour from Ponder's End, wine from Cobham ("A white, a rosé, and a sparkling Pinot Noir") mushrooms from East Ham, honey from Tower Hill, trout from Sion Park and a new Konstam porter from Battersea.

The Metropolitan Line doesn't come as far as Canvey Island, but Rowe includes it as it has the "spirit of London". Anyway, it is here that much of Konstam's fish is to be sourced. Rowe and I climb from a jetty on to one of a pair of trawlers run by Martin Yowarth and with Martin's nephew Alex we set out into the Thames Estuary.

We chug into the mainstream; an oil terminal to one side, the Isle of Grain power station to the other. It's not exactly picturesque but apparently the muddy Thames is now clean and teeming with creatures. Yowarth - fishing both pelagic upper waters and demersal deeper waters - finds bass, cod, whiting, plaice, turbot, dover cuttlefish and even lobsters, which although not the world's fattest, shift at £8 a kilo. There are even a few exotics, such as red mullet. "It's a global warming thing," says Yowarth, adding that he has also taken uglies such as angler fish and a 90lb conger eel from the depths. "I got my mate to wrestle it," he says. But at this time of year, the fishing is primarily a case of sprats and herrings. Yowarth watches the "fish finder", the scanner, which shows up the "marks" or shoals of fish. With colleague Andy Mazriel in the other trawler, he links a net between the boats and we all steam eastwards. "This is why we like it," says Yowarth, over the screeching of seagulls. "It's hunting. Primordial stuff." After an hour or so, the boats come together, bringing in the silver harvest.

Rowe plans to fry the sprats in a beer batter, probably made with Young's Champion Lager. And he's experimenting with pickling herrings, trying out a vinegar escabèche, where the fish soften in an acidic marinade. He fries some fish for lunch: sprats rolled in flour and eaten whole, herrings split, and rolled in oats. "It's northern European cuisine," says Rowe. "I'm fed up with that roast aubergine and courgette mock-Mediterranean thing."

The Konstam conceit certainly chimes with the current trend for locality and against excessive food miles. Rowe has discovered that London's "food footprint"- the area that feeds the city - is 25 times its actual size. It can't be self-sufficient but could be a lot more sustainable. "I think Hugh Fearnley-Whittingstall is doing a great thing, but it's a bit of a middle-class wet dream to go to the country and live off the land," says Rowe. "In places like King's Cross, with a dense population, it's a different matter."

It's not even the case that London's food is polluted, says Rowe. "I've been talking to Defra, as well as a toxicologist and a nutritionist about that," he says. "They've done work on allotments where they found traces of lead within 25 to 30 metres of a large road; after that it drops off. Most you can wash off, and it's not at a level that would effect the body."

We steam back to Canvey Island with 20 boxes of sprats and herrings: a good catch. And, I can testify, they are as clean and fresh as any fish from the North Atlantic. What next for Rowe? "I'd like to find someone who can source me some squirrel."

Konstam, 2 Acton Street, London WC1, tel: 020 7833 5040. The Urban Chef starts BBC2, 5 June at 6.30pm