Open Monday through Friday, 11:45 a.m. to 2:30 p.m. and 5:30 to 10 p.m.; Saturday, 6 to 10 p.m. AE, CB, MC, V. Reservations required.

Prices: Dinner averages $40 to $50 plus drinks, taxes and tip.

Imagine Belon oysters, fresh from Maine, very saline, barely heated through to plump them and intensify their oyster essence. Imagine them grouped with geometric precision on a large flat white plate, each covered with a leaf of dark green lettuce cooked just so it began to wilt and mold to the oysters, but still ever so slightly crunchy. Imagine over all a cream sauce only thick enough to cling to the lettuce leaves a gauze, the slight acid of wine cutting its richness. Imagine a few matchstick cuts of beige mushroom adding the faintest tinge of earthy taste and firm texture. Call it huitres tiede en salade aux champignons et laitues, and charge $7 for it as an appetizer. It is at Le Pavillon.

Le Pavillon is an uncommon restaurant - uncommonly good, uncommonly original, uncommonly expensive - and so I have reviewed it in an uncommon manner, visiting it five times (never anonymously, I am sorry to say) and spending a day in the kitchen to see how it does what it does.

First, you must understand that this is not Le Pavillon of last year. The subterranean dining room is the same, with its overblown banality but well-spaced tables and sumptuous floral bouquets. The dining room staff remains from the restaurant's classic cuisine days. But since late last year Le Pavillon's kitchen has undergone a culinary coup d'etat, and nouvelle cuisine reigns. Other French restaurants serve nouvelle cuisine; only at Le Pavillon does it occupy the entire menu.

I can't tell you precisely what nouvelle cuisine is at Le Pavillon, because next week it will be something different.It is change; it is experimentation; it is a chef's expression and contemporary interpretation of classic cooking techniques. It is also a quest for purity that is every bit as expensive in terms of time and resources as any search for truth.

It starts with extraordinary ingredients: turbot and frog's legs flown in fresh from France, herbs from nearby greenhouse, vegetables and fruits at their seasonal heights. But several French restaurants in town do that. Next Le Pavillon sculpts the food. Julienned vegetables are exact, each baton of carrot indentical to each baton of celery. Fruits for a sherbet and fruit plat are carved with great tediousness (and appalling waste) to fit a precise plan. Tile cookies so thin as to be nearly transparent, are spread into fragile rounds and curved over a broomstick.

An order comes for a salad of watercress, lobster, quail and truffles ($9.75, an appetizer). The watercress, carefully picked leaves with no coarse stems - is arranged in a circle as if it were a life's work, each leaf equally worried over, while the waiter frets about his hungry diner. Tiny circles of lobster and truffle are arranged according to some invisible dictum. Rare, freshly cooked quail breast is sliced at the time of each order and overlapped on top. The salad is beautiful, fragrant with olive oil and a heavy sprinkle of black pepper. And it illustrates that at Le Pavillon the cuisine is all.

Cold dishes are garnished as if prepared for a culinary art contest. A salad of millimeter-perfect concentric circles is sprinkled very lightly with chopped egg - a pinch here, a pinch there, a few grains removed from that spot and moved elsewhere. Truffle strands are arranged with as much concentration as if the garde manager were playing pickup sticks. Tiny parsley leaves are placed as if they were pearls on a brooch. And - voila! - the food is beautiful, the kind of delicate beauty one identifies with Japan. When a dish is set before you at Le Pavillon your pleasure is as much in the looking and smelling as in the tasting.

Seafood is steamed in its own juices in a non-stick pan; in the nouvelle cuisine fashion, it is cooked so briefly that it retains a soft velvetiness. Cream sauces include essences of the meat or fish they join and are thickened only by a reduction of the cream and a light liaison of egg yolk. Brown sauces are actually golden, thin and translucent, with no thickening. Vegetables are so lightly cooked that they lose almost none of their raw crunch. The garlic cloves that impart only the faintest perfume to the roast lamb are in fact too raw to eat.

Nouvelle cuisine celebrates contrasts - a soft pink pillow of salmon mousse with firmer white frog's legs and the sour grassiness of sorrel, vegetable purees touched with tartness and grainy texture of pears. Colors, texture and flavors interplay, often with some surprise. Yet, there are themes; this week's lobster mousse echoes last month's chicken liver mousse. The golden color of one meat sauce reflects another. Concentric circels become an edible logo. One learns to recognize the chef's cream sauce, his particular style of terrine.

As with any experimentation, there are errors. The Muscovy duck breast, cooked rare and sliced, was too tough for that method of cooking and its gaminess eclipsed the subtle sauce which, being unthickened, left just a wash of perfume on the meat. The trout filets stuffed with salmon mousse might be considered superb in another context, but matched against the other appetizers, it was too much a monotone of texture and flavor.

But it is so much easier to find successes on the short menu. The raw salmon is sliced to a coral haze spread across the plate, with a sprinkling of olive oil, scallion, mustard seeds and pepper. Lobster is steamed - barely - and brilliantly matched with crisp asparagus tips. Whatever meat is teamed with cabbage that day is going to be a revelation. Even a simple filet of beef in mustard cream is rich and sharp, in tantalizing balance. This is food that those of us who continually talk and dream of food will find necessary to try, to add to our experience.

For many, though, it will not be worth the cost. Appetizers average $7, main courses $18 at dinner. Even desserts hover around $7.Prices are less at lunch, but one would expect to spend more than $20 a person even then. The way to enjoy dinning at Pavillon most is to order a fixed-price dinner ahead of time, to attend one of the monthly gourmand dinners, or to establish a price with the maitre d'hotel when you are seated, and have him bring you a succession of small portions, half a dozen courses. It will cost you at least $40 a person just for the food. Add to that wine (the list is young, weighted with very high priced wines, with a few choices under $10 that don't live up to the food, and little between $10 and $25). Including tax and tip, dinner for two can easily reach $150.

The pity is that the dinning room does not do justice to the food. You also may find that the portions are small for your appetite. The desserts are the weak part of the menu, although the white chocolate mousse in a tulip-shaped cookie cup (unlike other courses, large enough to share among two or more) has become locally and justifiedly famous. The apple tart, cooked to order, is soggy; the sherbets are not homemade. The best ending is the plate of tile cookies that comes after dinner.

Le Pavillon is limiting reservations to no more than the kitchen can comfortably handle even though the dining room may not be full. The diner waits for the food rather than vice versa, so you should plan on several hours for dinner or a very long lunch. Cuisine is all, and it is constantly evolving under this new young French chef, Yannick Cam. And people who are serious about the pleasures of the table will want to follow every episode.