The leading chef in the world now almost certainly is Freddy Girardet of Switzerland. On every rating list, his quite-small Restaurant Girardet, in the town of Crisier near Lausanne, rates about as high as a restaurant can get -- three stars, four red toques, 19 out of 20 points. Confirming a reservation at one of Girardet's 20-odd tables is about equal in magnitude to procuring an invitation to dinner at the White House.

When I visited Girardet's restaurant, I was overwhelmed by the sheer brilliance of the meal he served -- course after course, dish after dish, each seeming to surpass the others in inventiveness, although each was utterly simple in concept. The marvel, each time, was the unusual -- but exactly right -- juxtaposition of the ingredients and the startling tricks that had been played with them.

The most memorable dish was a grilled fillet of whitefish resting on a sauce of pureed onions flavored with a touch of raspberry fruit vinegar and Tasmanian honey. Alongside the fish was a small mound of reduced and concentrated tomato butter. There was no other decoration. With each mouthful of fish, you took a little bit of the sauce and spread on a tiny amount of the tomato butter. The delicate combination of acid and sweetness was dazzling. FREDDY GIRARDET'S ONION SUACE WITH TOMATO BUTTER FOR FISH (4 servings) Tomato butter: 2 cups coarsely diced, peeled top-quality tomatoes 5 tablespoons unsalted butter Salt and freshly ground white pepper Onion sauce: 8 tablespoons unsalted butter 8 cups finely sliced yellow onions 5 to 6 tablespoons fuit or herb vinegar 1 teaspoon honey (use a variety such as Tasmanian, aromatic from flowers or herbs) Salt and freshly ground white pepper Fish: About 2 pounds fillets of white-fleshed fish (such as sole, flounder or striped bass) Unsalted butter Salt and freshly ground white pepper

Place tomatoes in saucepan or saute pan and, stirring frequently, heat to gentle bubbling. Keep heating, stirring regularly, with pan uncovered, until tomatoes form a thick puree and volume has been reduced to about 1/2 cup, in about 40 minutes. The puree does not have to be absolutely smooth.

Blend 5 tablespoons butter into puree, mixing thoroughly. Season with salt and white pepper to taste. Place butter in a storage jar and refrigerate until cooled.

While tomatoes are cooking, melt 8 tablespoons butter in saute pan over medium heat. Spread in onions and saute, stirring regularly, until reduced to about 2 cups, in about 45 minutes. Puree onions with a food processor (or alternative); strain through a fine sieve. Place in saucepan over low heat. Add vinegar, teaspoon by teaspoon, to taste -- usually 15 or 16 teaspoons. The sauce should have an acid tang, but still be delicate and gentle. Add honey, 1/4 teaspoon by 1/4 teaspoon, to taste -- usually 1 teaspoon is enough for a faint, distant touch of sweetness. Add salt and pepper to taste. Finished sauce can be kept warm, covered, or refrigerated for later reheating.

Broil or grill fish by your favorite method and heat serving plates. Spread 4 to 5 tablespoons onion sauce on each plate and carefully place individual servings of fish on top. The sauce should be visible around the fish. On one side of the plate place a mound of 1 to 2 tablespoons tomato butter. The diner dips each forkful of fish into a little sauce and a bit of tomato butter. The result is pure Girardet magic.