THE IDEA of eating raw fish is becoming more acceptable to American Gourmets. First on the list is smoked salmon, which is so firm and beautifully opaque that some aficionados do not realize it has not been normally "cooked", but only cured for several days in warm wood smoke.

Then there is the famous Peruvian dish "ceviche", in which raw fish is "cooked" to a firm opaqueness by being soaked for a few hours in lime juice. (This trick now has been picked up by French nouvelle cuisine chefs, who serve thin slices of raw fish "cooked" in lime juice and sprinkled with whole green peppercorns and black caviar.) And there is the Japanese "sashimi -- beautifully shaped cubes and rounds of raw fish to be eaten with chopsticks and dipped into sauces of soy, rice vinegar and "wasabi" (green horseradish).

But perhaps the best technique is the Scandinavian method of "gravning," pickling fish with a precisely balanced mixture of fresh dill, pepper, salt and sugar. It most often is done with salmon, when the final result -- firm, thin slices smooth and silky as velvet, magnificently aromatic -- is called "gravadlax." It makes a superb appetizer. Slightly bigger portions can be served as a main course for lunch or supper.

Outside Scandinavia, the best North American gravadlax I had eaten is in Toronto, at one of its top restaurants, Winston's. Owner John Arena and his customers are so devoted to gravadlax that he keeps on his staff a Swedish chef, Rolf Ronberg, for the regular "laying down" of huge specimens of the finest king salmon.

Ronberg makes batches of 18 pounds at a time, but I found that I can do as little as 4 pounds or even 2 pounds with a little extra care) with superb results. Sometimes I substitute a center cut of king mackercel, which is normally less than half the price of salmon. The dish is properly called "gravad makrill."

You must buy a good solid center cut, with the skin on but all scales and fins removed. Have the fish market carefully split the fish in half and remove the center backbone, then put the halves together again. When you get it home, you should complete the job by pulling out and discarding all other small bones. The only other essential is that you have plenty of fresh dill. Do not attempt this recipe when dill is out of season. Chef rolf rongerb's cured raw salmon of MACKEREL (16 appetizer servings) 4 pound center-cut salmon, or king mackerel, scaled and boned, skin left on 1/2 cup coarse crystal sea salt 1 1/2 cups sugar 3 handfuls small spruce sprigs, optional 4 large bunches fresh green dill 1/4 cup whole white peppercorns, coarsely cracked and crushed in a mortar Mustard sauce: 6 tablespoons fine Dijon mustard 6 tablespoons top-quality olive oil 2 tablespoons sugar 2 tablespoons red wine vinegar 6 tablespoons finely snipped fresh dill Salt and ground white pepper

Have ready a ceramic or glass dish large enough to hold fish (Dish must not be metal.) Pick over fish for bones, wipe flesh dry (do not rinse it) and hold it aside. In a bowl thoroughly mix salt and sugar; set aside. If you have spruce sprigs, place a third of them neatly across the bottom of the dish. Sprinkle a third of the salt-sugar mixture over sprigs. Rub some of the mixture on the bottom skin of the fish. Arrange a third of the dill in the dish (snipped coarsely), making sure that the stalks radiate outwards and that the green fronds are all at the center, so that they will have good contact with the fish. Sprinkle a third of the cracked pepper on top. Place lower half of fish in dish, skin side down.

Rub surface of fish with a little of the salt-sugar mixture and repeat layers of spruce sprigs, the salt-sugar mixture, dill (snipped more finely) and pepper. Rub flesh side of the second half of the fish with salt-pepper mixture and place in dish, skin side up. Using your hands, press fish halves together.

Lightly rub the top with salt-sugar mixture and repeat layers of spruce sprigs, salt-sugar, dill and pepper.

Cover loosely with plastic wrap or wax paper, and then cover tightly with aluminum foil, being careful not to let the foil touch the fish. Where the fish sticks up under the aluminum, place a fairly heavy board on the top of it to press down on the fish. Put fish in refigerator and leave it strictly alone for 10 hours. During this time the fish will leach out most of its water and will sink down in the dish.

At the end of the first 10 hours, remove foil. If there seems to be too much liquid in the bottom of the dish, remove part of it with bulb baster, but be careful not to remove any of the aromatic seasonings, or any large amount of sugar or salt. Now carefully remove seasonings from top of fish and hold aside. Using hands, carefully turn fish over, so that the top skin goes to the bottom. Spread seasonings back on top and baste with dish liquids. Seal and wrap fish as before, replacing the board. Refrigerate, undisturbed, for another 10 hours. Then turn fish again exactly as above. Seal it again and refrigerate for 12 hours. Turn fish again and re-seal, but this time do not replace board. Fish is ready to serve after 12 hours of refrigeration.

During the final few hours of marination, whisk together mustard, oil, sugar, vinegar, dill, salt and white pepper, all to taste. Place in pretty serving dish.

When the time has come to serve the fish, gently scrape away seasoning from the part you are going to slice (but not from other parts,) then lift fish out of dish into a carving board. Wipe off marinade, pepper, salt and sugar. With a very sharp knife, slice fish thinly across grain, usually downward toward the skin. Do not serve the skin. Serve fish slices on ice cold plates with mustard sauce.

Here's a bonus: a dessert light and cold as Alpine snow.

My Parisian friends report that the latest fashion in cafe desserts is one of the bright stars of the turn of the century --la mousee au chocolat blanc . When white chocolate mousse is set before you, a pure white fluff in a tall-stemmed glass, as cold as the snow on the peak of an Alpine mountain, it seems to be the perfect cooler for the hottest of summer days.

In San Juan, P.R., a few weeks ago, I was served a white chocolate mousse of quite extraordianry lightness and smoothness, prepared at the Caribe Hilton International Hotel by excecutive chef Emil Graf. He taugh me to prepare his version of white chocolate mousse, which involves little of the work of classic mousses.

Graf's secret is the use of Swiss Toblerone chocolate bars, which melt like a breeze and have a texture balance of honey-nugat and nuts. Starting with this base, he whips up a near-perfect mousse in about 10 minutes. EMIL GRAF'S WHITE CHOCOLATE MOUSSE (4 servings) 2 (3-ounce) bars Toblerone white chocolate 1/3 cup whole milk 1 cup heavy whipping cream, very cold 2 large egg whites 1/2 teaspoon lemon juice Small bar dark semisweet chocolate

Put serving containers and the storage bowl into freezer and chill to ice cold.

Break white chocolate bars into sections, cut each section into about 3 smaller pieces. Heat milk in the saucepan and, when it is hot but not boiling, stir in chocolate. Heat, stirring occasionally, until chocolate is melted. Do not let it get near boil. When it is smooth, remove from heat and let cool, uncovered, stirring accasionally.

Meanwhile, beat cream until it forms stiff peaks. Spoon it into storage bowl and hold in refrigerator. Beat egg whites until frothy, then beat in lemon juice. Continue beating until whites form stiff peaks.

When chocolate is cool, use rubber spatula to lightly fold it into whipped cream. Fold in egg whites. (The final lightness and smoothness of the mousse depends largely on the skill of your hand at this point. Do not overdo folding so that the lightness is reduced, yet be sure the ingredients are well combined.

Spoon mousse into serving dishes and chill in refrigerator, covered, until very cold, in 3 to 4 hours. Just before serving, decorate with shavings of dark chocolate.