At a recent dinner in a Brussels hotel, we were offered a duck breast encased in a firm envelope of foie gras. While we were pondering just how the duck and goose liver had been cooked together without the latter becoming a molten mass, word got around the table that Georges Pralus was in town introducing the kitchen staff to the wonders of vacuum cooking.

In vacuum cooking, food is covered by a thick plastic wrap that is sealed after virtually all of the air surrounding that food has been removed. (Much supermarket meat and cheese is wrapped in this way.) Although this type of vacuum packing was developed as an effective means of minimizing the deterioration of fresh foods, about 10 years ago Pralus began exploring the possibilities of cooking foods in the vacuum-sealed bags.

Pralus' immediate inspiration for his experiments were the brothers Troisgros, the renowned restaurateurs of Roanne (just northwest of Lyon in France) who were seeking innovative ways to store, cook and serve fresh foie gras for their restaurant. It was during this period that he created the duck-breast-and-foie-gras recipe and discovered immediately that when cooked within the vacuum-sealed bag in a specially designed steamer, none of the natural flavors of the ingredients were lost.

"In fact," said Pralus in ebullient French when we interviewed him in the hotel's kitchen after our meal, "I find that I need much less seasoning" when cooking in the plastic wrap. "This is a wonderful method for people on salt-free diets because the natural flavors of the foods are much more assertive," he said. "Since the foods all cook in their own juices, the resultant liquid also makes an ideal base for a sauce."

Pralus' next step was to take the French technique of cooking en papillote and replace the traditional greased paper bags with the plastic. We tasted a whole fresh salmon steamed in its own juices with this method and found it to be delicious. (Had we not known differently, we would have guessed it had been expertly poached.)

Pralus also showed us a partially cooked rack of baby lamb sealed in a bag with julienned vegetables, which had been prepared and refrigerated since earlier that day (but could have been stored in that state for up to five days). He finished off the dish by steam-cooking it an additional 10 minutes and then setting the lamb under a salamander for an instant to brown it. The result was very tender, pink baby lamb chops surrounded by a thin crispy border of fat, and vegetables that were crunchy and full flavored.

"And," Pralus informed us with one of his enthusiastic smiles, "meat cooked this way loses only 8 percent of its weight while roasted meat loses about 20 percent. Another advantage is that vacuum-cooked meats maintain their original coloring since the process eliminates their exposure to air."

Prulaus' enthusiasm for the technique is quite contagious, and the kitchen staff gathered around him nodding in agreement as he spoke. "Vacuum-cooked meats are also more tender," he continued, "since the collagen a gelatinous substance found in the connective tissue dissolves more slowly and the muscle fibers do not contract during cooking."

"You know," he added proudly, "the most famous restaurant for cooking chicken in France -- the Auberge Bressannde in Bresse -- uses my techniques, and many of the best restaurants in France now consider vacuum cooking an essential part of their repertoire, and a means for inventing dishes that were not possible before."

The advantages of the vacuum cooking for a large hotel kitchen are perhaps more practical. Michel Theurel, head chef at the Brussels Hilton, is extremely optimistic about the introduction of the technique into his kitchens. He feels that the technique will enable him to maintain freshness of ingredients more effectively and thereby eliminate waste.

"We can also take advantage of a more regular preparation schedule," he claimed, "with no rush hours just before meal time. This will improve our quality as well as our staff morale. In addition, the chances of food being improperly cooked are almost entirely eliminated since the cooking time in the special steamer can be quite precisely determined."

And the disadvantages? "Well, we understand the limitations," said the chef. "Obviously you can't bake bread with this technique, or get a nice brown crust on a roast. But it's a great system, and the only real complaint we might have is that it can be a nuisance to snip open all those bags. But then again, we don't have to clean the oven . . ." Vacuum Cooking at Home

The vacuum-cooking technique has some very interesting applications for the home cook. The Dazey Seal-A-Meal Co. sells a compact, easy-to-use, and inexpensive machine that vacuum-seals small quantities of food in plastic pouches. (The vacuum-sealer, Model SAM-3, retails for about $30 and is available in most housewares departments, or write Dazey Products Co., One Dazey Circle, Industrial Airport, Kan. 66031.) These plastic pouches are not only efficient for refrigerating and freezing foods, but also may be put to creative use for preparing foods by steaming, boiling or microwave cooking.

Paula Wolfert, author of the highly acclaimed "The Cooking of South-West France" (Dial; $24.95), first learned to cook in vacuum-sealed bags from French chefs while researching her cookbook. Although they were using very expensive professional equipment, she began experimenting with the technique in her home kitchen by using multi-layered, plastic-wrap packets or the vacuum-pouches. She was delighted with the result and quickly concluded that "flavor, juiciness and nutrition are all retained better than in any other method I know for preparing chicken breasts, mussels, shrimps, salmon, and white-fleshed fish."

Here is a recipe from Wolfert's book that uses the vacuum cooking technique and may be prepared either with or without the machine, with delicious and elegant results. Following these are two simple recipes from the Dazey Seal-A-Meal recipe booklet that reveal how the "scorch" factor is eliminated with pouch cooking. STEAMED SALMON WITH COOKED EGG SAUCE (4 servings)

Wolfert found the inspiration for this recipe from Toulouse chef, Lucien Vanel. 2 salmon steaks (about 10 ounces each), cut 3/4 to 1 inch thick Salt and freshly ground pepper 4 teaspoons unsalted butter, cut in 4 even pieces, softened 2 large eggs, set under running warm water for 1 minute 3/4 teaspoon dijon mustard 2-3 tablespoons fresh strained lemon juice 1/4 cup mild olive oil 1/4 cup peanut oil, preferably french 3 tablespoons heavy cream 1 small ripe tomato, peeled, seeded, chopped, and drained (about 1/4 cup) 2 tablespoons chopped fresh parsley 1 tablespoon small capers, rinsed and dried 2 tablespoons chopped fresh chives 1 1/2 teaspoons chopped fresh basil leaves (optional)

Trim bones and skin from salmon. Season steaks with salt and pepper and coat with a thin film of butter. Wrap each steak water- and airtight in 3 or 4 layers of heatproof plastic wrap such as Saran Wrap, or wrap airtight using a vacuum packing appliance. Refrigerate until ready to cook.

To make the sauce, cook water-warmed eggs in simmering water 4 minutes. Remove and refresh under cold running water for 1 minute. Carefully scoop out soft-cooked yolks into small mixing bowl or top of double boiler. Set aside whites.

Set bowl or top of double boiler over hot but not simmering water. Mash egg yolks with spoon until smooth and pastelike. Beat in mustard and 1 tablespoon of the lemon juice, whisking until the mixture thickens. Very gradually add combined oils using a beaker-type container; add 1/4 teaspoon oil at a time, whisking constantly. An emulsion should form -- just as it does when you make homemade mayonnaise. Remove from heat if the mixture at any time appears about to curdle. (If this happens, dip the bottom of the pan into a bowl of cold water or quickly whisk in a spoonful of cold heavy cream.) It takes about 5 minutes to add all the oil. Remove from heat and add heavy cream.

Press whites through medium sieve with back of a spoon. Fold whites and remaining ingredients into sauce. Season and reserve. Sauce can be prepared early in the day and reheated very gently.

To steam the fish set steamer rack over 1 1/2 inches boiling water in large saucepan. Steam 4 minutes per side. Do not overcook; salmon continues to cook while waiting to be unwrapped. If the salmon is in vacuum-sealed packets, you have the option of dropping the packets into simmering water to cook for 7-8 minutes. If the packets float to the top, weight them down with a plate so that the water covers them entirely.

To serve, carefully unwrap the salmon and place on individual heated serving plates. Top with lukewarm egg sauce. WELSH RAREBIT (6 servings) 1 pound shredded sharp cheddar cheese 3/4 cup cream or half-and-half 1/2 teaspoon salt 1/2 teaspoon dry mustard 1/2 teaspoon worcestershire sauce Dash cayenne pepper 6 slices toast

Place all ingredients except toast in 8-by-12 inch pouch and seal. Cook in boiling water until cheese melts, about 15 minutes. Knead gently to mix, then open pouch and pour over toast to serve. FUSSLESS FUDGE SAUCE (Makes 1 cup) 6-ounce package semi-sweet chocolate pieces 1/4 cup half-and-half 3 tablespons water

Place all ingredients in an 8-by-9 inch pouch and seal. Cook in boiling water until chocolate melts, about 15 minutes. Remove from water and let cool for a few minutes. Then gently knead with fingers or back of wooden spoon to blend all ingredients. Serve warm over ice cream.