The gospel according to St John

Fergus Henderson loves his food. It's a visceral thing. The London chef who has turned offal into the hottest dish in town, has a lip-smacking passion for his produce that is a million miles away from the nervous whinnyings of foodies. The sole end of his art, he explains, with a little hip-hooray gesture, is "big, happy feasting".

Try telling that to the critics. Since 1995, when Henderson opened St John, a converted smokehouse near Smithfield market, page after esoteric page has been filled on the faintly perverse pleasures of St John signature dishes such as roast bone marrow and parsley salad, duck hearts on toast and rolled pig spleen. Only the most refined palate, it is frequently implied, can truly appreciate tripe and onions. Henderson, however, takes a frankly evangelical stance and is now bringing his message to the masses in a cookbook entitled Nose to Tail Eating: A Kind of British Cooking. "There is a set of delights, textural and flavoursome, which lie beyond the fillet," he urges in the book's foreword. "Nothing," he adds, blinking moleishly behind round spectacles, "could be as lip-sticking as some of these extremities with their wonderful half-flesh, half-fat quality." This is the problem with restaurant cookbooks. Sometimes you just have to be there.

In the kitchen of Henderson's Covent Garden flat, there is a great big wooden bowl full of fruit on display, but this is fooling no one. An ox- blood coloured metal cupboard some 8ft long and 6ft high may well be stuffed full of muesli and sun-dried tomatoes, but it looks like it was custom built to conceal carcasses. The kind of kitchen shears that could sever a man's wrist glint from a rack. Henderson and his wife, the New Zealand chef Margot Clayton, are, on the whole, more irritated than amused by their Sweeney Todd image, and there is indeed no rational explanation as to why the preparation of innards should seem more sanguinary than roasting a chicken. Henderson holds his meat in high professional regard, a kind of matador's respect for the bull, and is pained by the indignity of bloodless supermarket meat gussied up for the squeamish consumer in "little pink hairnets".

"We saw a sheep being killed in New Zealand. and then it all kind of made sense," he recalls. "It was amazingly straightforward. The man doing it was obviously very good at his job. He just whipped off its coat like he was a coat-check person, opened it up and pulled out the entrails. You could still see the last meal of grass being digested. It was almost alarming how this animal seemed designed to be butchered," he goes on, with a kind of awestruck wonder, "as if it had been made for the table." But the hardiest hunter-gatherers have their off days. Henderson tells a heart-rending tale of a bucket of snails gathered for the pot during a family holiday on Tiree in the Hebrides. "Snails have to be starved and purged before they're eaten," he explains, "so we left them in a bucket covered in pierced clingfilm. We seemed to be watching these poor captive snails leaving trails of poo on the sides of the bucket for days. Eventually, one of the party cracked and liberated them." The rescue mission was carried out under cover of darkness, but the finger of suspicion still points at Henderson. "It was the trails of poo that did it," he confesses. "I'm happy to grapple with anything but it was that poo, their dying gesture, leaving their last mark, that would have broken the strongest of spirits."

It is absolutely not Henderson's intention to turn crispy pig tails into the new ciabatta. He has a fastidious horror of food fads, which is reflected in his own, steady-as-you-go style of cooking. The more outlandish cuts on the St John menu are carefully balanced by the kind of broths and stews and briskets that wouldn't scare the horses at Simpson's on The Strand. "I like to think of what I'm doing as permanent British cooking," he says. "It's not olde-worlde and it's not `modern British', but it makes sense where it is and how it is, here and now." He also refuses to indulge in the current vogue for writing menus as if they were lonely hearts columns (you know the sort of thing, "tender young lamb on a rumpled bed of negligee vegetables") and presents clients with bald choices like pigeon and peas or beans and bacon. At times this minimalist rigour has itself teetered on the edge of affectation (a starter of plain boiled egg and raw carrot was, briefly, the talk of the town), but you have to applaud the intention.

Henderson's uncluttered aesthetic was inherited from his father, Brian Henderson, the architect responsible for Gatwick airport. His mother also trained as an architect as did his sister, but food was the family's consuming passion. "Eating out featured to an almost fanatical extent in family life," he recalls. On gourmet holidays in France, the teenage Fergus would keep diaries with detailed drawings of his dinners, and the two disciplines - art and eating - somehow fused in his mind. He followed the family profession and qualified to RIBA part II from the Architectural Association, but somehow, he remembers, "my designs always ended up as recipes. I was doing recipes for buildings."

When you look at St John, which Henderson designed himself, you can see what he means. Stark white walls and austerity paper tablecloths match the no-nonsense menu. The atmosphere is added as the evening wears on. "There is this mysterious process between the materials you start out with and what you end up with," he explains. "Certain foods, and the way you serve them, make people behave in a certain way, in the same way as architecture affects them."

Whether this works quite as Henderson intends is debatable. St John has a big following among City blokes who seem to regard eating entrails as a kind of test-your-own-testosterone exercise. ("I'll raise you a spleen" - "OK, I'll raise you a gizzard.") Maybe it is just comfy childhood memories of Desperate Dan and his famed cow pie, but since the BSE scare, there has been a rush on the roast marrow bones, which, to the evident disappointment of some diners, are perfectly legal. "I can't help feeling that it's not because they're thinking `yum yum, bone marrow, that'll be tasty', but because they think its some kind of feat of macho bravado. But that's not what it's about. It's certainly not what I'm about. If anything it's much more jellies-and-tripe than rip-a-limb-off stuff. I mean," he tails off, "just look at me ... I'm not ... oh, um dear. Do you know?" Henderson's wife Margot Clayton, grins supportively at this point.

The two met seven years ago. Henderson, who, while still an architecture student, would rustle up 200 covers for Sunday lunch at Smith's Restaurant in Covent Garden, had finally made the leap from drawing board to stove and was running his own eating and drinking club, 17 Mercer Street. Clayton had made her name in London as the chef at Portobello Road's First Floor restaurant. A kitchen porter who worked first with Margot and then with Fergus, saw compatibility in their cooking styles and played culinary Cupid. In 1993, they opened the French House Dining Rooms together. Business preceded romance by exactly one day.

"We had our first business meeting and I went away thinking `Great, we've discussed all the important things' and Fergus went away thinking `Damn! I didn't get there'" reports Margot with huge satisfaction. "By the weekend we were lovers and we never spent a night apart again.

"When we started together, I just couldn't believe Fergus. I mean, great menus, lovely food, but he didn't know the first thing about cleaning or organising supplies ... " "But we had just met," interjects Fergus, "there was passion in the kitchen."

"And he knows a lot more now," finishes Margot. It is an excellent double act. These days Henderson chefs at St John while Clayton cooks in the morning at The French House and organises the rest of her time around their son Hector, five, and daughter, Owen, three. A third child is expected in June. "I suppose it might seem a bit ridiculous that we don't have more restaurants by now," says Clayton a touch defensively, (although the question has neither been asked nor thought of) "but it's a matter of priorities."

These were quietly reshuffled three years ago, when, aged 32, Henderson was diagnosed as suffering from Parkinson's disease. Only when your attention is drawn to the condition do you notice that his left side is fractionally less animated than the rest of him. "Initially, for a moment, when we first found out about it, it wasn't great," he says with characteristic deprecation, "But apparently I got it in the right decade. Everybody is doing wonderful things in the neurological field right now and the medication is great. It's not something I would try to hide. I think it may be important for people to see that your life doesn't stop because you've got Parkinson's. Before I started the medication I would get very dozy, so double shifts were tough, but now I take the right stuff, I go to see alternative practitioners and I have a physiotherapist who comes to see me and we jump around together, so in a way, I'm fitter than I was before."

It occurs, rather late, that underneath Henderson's gently shambling manner, there is a cool well of concentration, a Zen minimalism that goes beyond boiled eggs and brisket. "Oh, um yes," he agrees, "less is more, that kind of thing. Altho-ough ... " he adds,wonderingly, Pooh-bearishly, " ... sometimes a little more can be rather good"

`Nose to Tail Eating: A Kind of British Cooking' is published by Macmillan on 21 May, price pounds 20