Not the least of the many virtues of fish as a food is its adaptability. The methods of cooking various fish, like the sauces one can apply to them, are virtually limitless. Here is an unusual preparation, an interpretation of a recipe created by Pierre Gaertner at his Aux Armes de France in the Alsatian town of Amerschwilhr.

Fish, noodles and wine all are staples of the Alsatian diet. Gaetener binds them together with a hollandaise sauce. The result, to my mind and palate, is one of the best dishes in the entire fish repertory. No single part of the recipe is complicated, but as several last minute steps are necessary two cooks can work together to good advantage.

In Alsace, at Gaertner's restaurant, a meal might begin with foie gras or snails before the sole and follow it with cheeses, and pastry. For those who lack superhuman appetites, a good menu would be a clear soup or an artichoke, the sole, a green salad and fruit. SOLE WITH NOODLES (Serves 4) 4 fillets of sole (about 1 1/2 pounds), cut in half crosswise 4 shallots or spring onions, minced 2 tomatoes (use canned Italian-style), seeded and chopped 1/2 cup dry white wine (Riesling preferred) 1/2 cup fish stock* 3/4 pound flat noodles (freshly made, if possible) 1 tablespoon olive oil 1/2 cup creme fraiche* 2 sticks butter, clarified* 4 egg yolks 2 tablespoons lemon juice, or to taste Salt, pepper and cayenne pepper Preheat oven to 325 degrees and bring a large pot of water to a simmer for the pasta.

Wash and dry fish, removing any loose scales. Using butter, grease a heat-proof dish or gratin pan large enough to hold the fillets. Scatter shallots and tomatoes over the bottom, then lay the fillets over them. Salt and pepper generously. Mix fish stock and wine and pour it over the fillets. Cover with a butter sheet of waxed paper. Set aside while beginning sauce.

Mix egg yolks and 4 tablespoons water in a heavy-bottomed saucepan. Place over low heat and beat continuously until all the yolks have been incorporated into a frothy mixture. (Work on and off the heat as needed to keep the yolks from cooking.) Have clarified butter nearby, warm.

Bring liquid in fish dish to a boil atop the stove, then transfer it to the oven. Let fish poach for 8 to 10 minutes, or until firm but not flaky.

(If working in tandem, finish the sauce and cook the pasta at the same time. If working alone, finish sauce, then cook pasta.)

Working on and off the heat as necessary, beat butter into sauce base by driblets at first, then more rapidly as the sauce "takes." When finished, beat in salt, pepper and lemon juice (begin with a tablespoon) until the hollandaise has character. Set aside in a warm place, but off the heat.

Bring water to a rolling boil and add olive oil and a tablespoon or two of salt. Cook noodles until just tender, 3 to 5 minutes for homemade. Drain without cooling and mix with creme fraiche. Season with salt and pepper. Fill the bottom of a flat, heat-proof serving dish with the noodles. Place cooked fillets atop noodles and keep warm.

Strain sole cooking juices into a frying pan and reduce over high heat until less than 1/2 cup remains. Stir this gradually into the hollandaise. Taste and correct seasoning. Spread sauce over fillets and noodles and run under the broiler until top is glazed and brown. Serve at once.(FOOTNOTE)

* Notes: To make fish stock, ask for a fish frame (sole, flounder or rock will do) when buying the fillets. Place it in a pot with 1 quart of water, 1 onion, carrot and celery rib - all sliced - and small amounts of thyme, parsley, salt and a bay leaf. Bring to a boil, skim off scum, and simmer for 25 to 30 minutes. Strain. Remaining stock may be frozen for later use.

Creme fraiche may be purchased in a number of local specialty stores, but it is expensive. To make a workable substitute stir 3 tablespoons plain yogurt into 1 cup heavy cream. Cover and leave in a warm place such as a turned off gas oven for 6 hours or overnight.

To clarify butter melt it in a pan. Carefully skim off surface scum, then pour off clear liquid until deposit on bottom is reached. Discard deposit. Unsalted butter is easier to clarify than salted. (END FOOT)