Build a better mousetrap and, in these times, beat a path to the U.S. Department of Agriculture and camp on its doorstep. That at least is the path followed by David Liederman during the past 18 months.

Liederman is a young New Yorker who has developed a line of frozen sauce bases that should prove to be a boon to gourmet cooks and restaurant chefs alike. The idea grew out of his experiences as a cook in the famous three-star restaurant of the Troisgros brothers in Roanne, France. "We spent nearly 80 per cent of our time making and reducing the stocks that were used for sauces," he said during a recent visit to Washington to promote his products. (The six sauce bases are marketed under the Saucier label and are available locally at eight locations.)

"I realized it would save a lot of time if the sauces were already reduced and ready to use. When I got back home, I realized there was no competition." In fact, a company does make sauce bases for the restaurant and institutional field but not for retail sale. Using Liederman's system, the home cook prepares meat, fish or poultry by roasting, baking or another traditional method. Then an appropriate amount of the frozen stock is melted and easily transformed into a sauce.

The trick was not to devise secret recipes. The preparations Liederman wanted to make are engraved in the litany of French classic cuisine. He quotes Escoffier, the master chef who codified formal cooking: "Indeed, stock is everything in cooking, at least in French cooking . . . If one's stock is good, what remains of the work is easy; if, on the other hand, it is bad or merely mediocre, it is quite hopeless to expect anything approaching a satisfactory result."

Liederman made some adjustments, however, making himself vulnerable to attack from purists. Though reduced through long simmering, strong and flavorful, his products are not concentrated to the point of being meat or fish glazes - the literal translation of glace de viande or glace de poisson. The others are glace de homard (lobster), glace de voaille (chicken), demi-glace and Bordelaise.

Far worse, in the watchdogs' eyes of the U.S. government, were his desire to use a French name, his insistence on using only fresh ingredients and his refusal to employ artificial preservatives. It took his knowledge of the law (a lawyer, he has happily given up practice), persistence and, he said frankly, some "political pull" to get his product and its label approved and on the market. Its exact classification is still in doubt.

"I fought," he said. "I'm promoting a French product. I didn't want to call it 'beef bone-flavored vegetable stock.' Another company uses 'Sauce,' as part of a trade name, so they didn't want me to use 'Saucier.'

"I wanted to use fresh shallots. Fresh shallots aren't on the USDA approved ingredient list, but freeze-dried are. They said the fresh are dirty.I kept calling them until they said Okay. Then they killed me on leeks. It took two months to get fresh leeks through. I wanted to use celery out of a crate. They said it's dirty. They wanted me to use precut. But precut cost three times as much. I said I can't afford it. I'm a small business. They really make it so hard for small businesses."

Liederman also had to find a plant to produce his stocks. Several were incredulous at his plans to cook away so much wine; others were too busy. Some didn't meet his standards or match his volume needs. Finally he made arrangements with Howard Johnson's to use the modern facility at which they package and freeze products for their restaurants and hotels.

Packaging was another problem. "I'm promoting a system," he said. "People aren't ready to use these in bulk. They have to add something, so I want them to be able to pop them out of a cup with a volume that will match the recipes I've done. It costs twice as much to package them this way (in packages containing four two-ounce cups). It costs me money at this point. When I can sell to the consumer in a one-pound box I'll be all right.

"The hurdle is getting people to understand what it is."

That's what Liederman was doing in Washington recently. Promoting his product by showing people in stores how the flavored and reduced stocks could become sauces suitable for beef, fish or poultry; how they could be used in liquid - nouvelle cuisine - fashion or thickened with cream, farmer's cheese or vegetable purcees. He stayed on to prepare a Sunday luncheon for his hosts, Mitch and Linda Berliner, and several guests. The Berliners are produce and food retailers with stands at the Farm Women's Market in Bethesda and other locations. They also are the local distributors for Saucier, which will sell for prices (per four-cup box) from $2.95 to $4.50.

Except for making bread and doing some minor preparation work, Liederman did not begin the cooking until the guests arrived. He then concocted a fish course. The sauce, made rapidly once the fish was underway, contained equal amounts of demi-glace and glace de homard and was enriched with cream, butter and, as a bit of fantasy, two ounces of foie gras. As he claimed, it was a matter of heat and stir, with some minor seasoning adjustments.

"I don't think people should follow recipes exactly," he said. People who like to cook should like to cook, not go by set ideas and follow letter by letter the way somebody else cooks."

A rack of lamb anointed with Bordelaise sauce followed and dessert was a showcase for Liederman's modification of classic pastry cream, in which the flour was sharply reduced.

Here is a recipe David Liederman created for one of his Saucier products and two others he prepared for the Berliner's luncheon. Obviously, the sauces may be made with homemade stock reduced to a near glaze consistency instead of the commercial product.

Saucier products are sold at the following locations: Chevy Chase Supermarket; Neam's in Georgetown; Wagshal's Delicatessen; Charles of Capitol Hill; Suttler's Meat Market in Arlington; Bradley Food and Beverage, Bethesda; and Bloomingdale's. PORK CHOPS WITH DEMI-GLACE AND CREAM (4 servings) 8 loin pork chops Salt and pepper to taste 2 tablespoons unsalted butter 4 ounces (2 containers) frozen Demi-Glace 1 cuy heavy cream 2 tablespoons chopped parsley

Season pork chops with salt and pepper. Melt butter in a large skillet, add chops and saute over a moderate heat for 5 minutes on each side, or until cooked through.

While chops are cooking, melt Demi-Glace in a saucepan. When it begins to simmer, add cream. Bring to a slow boil, whisk until completely blended and cook for 5 minutes, stirring occasionally, until sauce is sufficiently reduced to coat a spoon. Season sauce with salt and pepper.

Place chops on plates. Spoon sauce over meat and decorate with chopped parsley. SEAFOOD MARYLAND (4 servings) Court Bouillon (see recipe) 4 large sole fillets Fresh basil leaves Freshly ground pepper 8 soft-shell crabs Flour Salt 4 tablespoons butter 1 pound bay scallops 4 ounces (2 containers) frozen Demi-Glace 4 ounces (2 containers) frozen Glace de Homard 1/2 cup heavy cream or creme fraiche 1 cup tomatoes, peeled, seeded and roughly chopped 2 tablespoons chopped chives

Make a court bouillon by combining 2 cups water, 1 cup dry white wine, a thinly sliced small onion, a tablespoon of whole black peppercorns, 1/2 teaspoon dried or 1 tablespoon fresh basil, 2 medium bay leaves and 2 tablespoons tarragon vinegar. Bring to a boil and simmer, partly covered, for 15 minutes. Strain and return to pot.

Meanwhile, cut fillets in half and place smoothside down. Sprinkle pepper on each, place on 2 or 3 basil leaves and roll up so leaves are inside. Fasten each with a toothpick. Mix 1 teaspoon salt with flour and spread on a plate. Melt butter in a frying pan large enough to hold the crabs in one layer (use two, if necessary).

Bring court bouillon to a boil. Add wrapped fillets, then cover pan and turn off heat. Dip crabs into seasoned flour and transfer them to frying pan after butter bubbles. Cook about 3 minutes on a side, turning once. Five minutes after fillets have been covered, add scallops, being sure they are submerged in the broth.

In a frying pan heat Demi-Glace and Glace de Homard. Bring them to a boil and stir in cream. Add tomatoes and stir and crush with a whisk. When tomatoes are softened, taste sauce and correct seasoning as desired. Keep warm while arranging plates. Place 1 crab on each plate, along with a fillet and a portion of scallops. Cover with sauce, then sprinkle on chopped chives.Serve at once.

Note: Liederman added about 2 ounces of foie gras to the sauce, emphasizing it was an optional ingredient. LINDA'S DESSERT (8 servings) 8 cooked puff-pastry bars, each about the size of a Napoleon 1 recipe light pastry cream* 2 tablespoons canned praline paste 1 tablespoon sugar 1/2 cup creme fraiche or heavy cream 1 quart strawberries, washed and hulled, sliced unless small 1 quart blueberries, washed

Mix praline paste with sugar and creme fraiche. Cut each puff-pastry shell horizontally into three pieces. Line the base piece liberally with pastry cream and top with strawberries. Line the second piece of pastry with praline mixture and top generously with blueberries. Place the remaining pastry piece on top.

It is easiest to assemble each pastry on the plate. Then if some filling spills over, no harm is done. To add whipped cream would make a visual impression, but would detract from the taste and flavors.(FOOTNOTE)

* Liederman makes his pastry cream with 3/4 cup granulated sugar, 4 egg yolks, only 2 tablespoons of flour and 2 cups of boiling milk. (END FOOT)