LIKE THEATER tickets and good summer sandals, shellfish is for the privileged classes or special occasions. So it's time to look around for substitutes--especially while it's salad season and most cooking mistakes can be rectified with a dab of mayonnaise or a blizzard of chopped parsley.

Cold fish is worth pondering. Cold haddock, red snapper, or even lowly flounder can set off currents of creativity in any mind turned toward food. Recalling the lordly salmon, one realizes that cold fish has possibilities. A succulent, poached cod fillet suits summer when it is served chilled, frosted with mayonnaise colle and decorated with the intricacy of an Italian fountain.

Fish preparation intimidates and perplexes many cooks. Some will tiptoe past the fish market, hoping that no one will ask them to go inside and make a definite decision. Fish is just too various--one of those things like sweetbreads that some people are happiest eating in a restaurant.

Sensing this timidity, cookbook authors recently have been turning out enormous tomes on fish that are too heavy to read in bed, and in most cases, too earnest to make the learning easy. Like everything in life, personal experience is the only way to learn.

Bless Washington for its fishy sophistication. At your fingertips are marvels you have yet to discover. Who knows what you'll love or hate until you've encountered it on a cold lettuce leaf . . . a solid slab of bluefish beneath a tall tangle of marinated vegetables--delicious for some, but others may prefer the milder taste of bass.

General cooking instructions are another matter fraught with prickly conflict. The Canadian theory says to cook fish or fillets for 10 minutes to the measured inch. Others say that's too long.

You will learn through trial and error how the fish looks when it flakes, or for a flakeless kind like monkfish, the right firmness when you prod it with a finger. In these cases, experience is the best teacher.

Currently the trendies eat their fish almost raw, and not just sushi. They demand that it come from the broiler or saute' pan with a translucent center which is synonymous with the purple center of a very rare sirloin steak.

Even if you are so chic that you replace framboise for cassis in your kir, fish in the following cold dishes will taste better if cooked through. Left rare, it is just too subtle.

Cooks can interchange most white fleshed ocean fish in these cold dishes. Try familiar ones first, and then experiment with species that are new catches in our waters--those that have become commercially viable by the large population of European chefs who have moved to America.

Flaked or cut in chunks to simulate shellfish, finny fish can perform as well in salads, and less expensively.

ESCABECHE OF FISH WITH GUACAMOLE (6 servings) 2 pounds fish fillets 1 1/2 teaspoons salt 1/4 cup flour 2 tablespoons butter 2 tablespoons vegetable oil 1 large onion 2 tablespoons lemon juice 2 tablespoons white wine vinegar 1/4 cup olive oil 1 teaspoon minced pickled jalapeno pepper Salt and freshly ground pepper Guacamole: 1 tomato, seeded and diced 1/4 cup finely minced onion 1 teaspoon finely minced fresh or pickled jalapeno pepper 1/4 teaspoon oregano 1 tablespoon red wine vinegar 1 large avocado, peeled and cut into 1/2-inch cubes Salt to taste Cherry tomatoes, romaine lettuce for garnish

Sprinkle fish with 1 teaspoon salt. Place the flour on a piece of waxed paper and lightly coat each fillet, patting to remove the excess. Heat butter and oil in a skillet and when the foam subsides, add the fish and brown lightly on both sides, about 8 minutes total for thick fillets, and 4 for sole. Peel onion cut in half lengthwise and slice in thin half-moons. Strew half the onions in a large shallow serving dish and arrange the fish on top overlapping slightly. Combine the lemon juice, vinegar, olive oil, minced pepper, salt and pepper and pour over the fish. Top with the remaining onion. Set aside at room temperature (or if the day is very warm, in the refrigerator). Allow the fish to marinate at least 3 hours, basting occassionally.

Combine the guacamole ingredients just before serving. Drain fish, discarding marinade. Top fish with the guacamole, spooning in a strip down the center of the dish so that the fillets and onions are not completely masked. Garnish with tomatoes and romaine. COLD FISH WITH SAUCE AMERICAINE (6 servings) 2 pounds firm white fish fillets Salt and flour for dusting 1/4 cup olive oil 1 small onion, finely chopped 1 clove garlic, crushed and minced 2 large tomatoes, peeled, seeded and finely chopped 1 1/2 tablespoons tomato paste 2 tablespoons minced parsley 1/2 cup dry white wine 1/4 cup fish stock or clam juice 1/8 teaspoon cayenne Salt to taste 2 tablespoons cognac 1 quart shredded curly endive Lemon wedges, black olives and pepperoncini for garnish

Cut fish into 6 even serving pieces, salt lightly and dust with flour. Heat olive oil in a skillet and saute' the fish on both sides until lightly brown, but not cooked through. Remove to a plate and add chopped onion and garlic to the oil in the pan. Saute' the onion and garlic for 2 minutes and add chopped tomato. Saute' for 3 minutes more, stirring frequently. Add the tomato paste, parsley, white wine, fish stock, cayenne and salt. Cover the pan partially and simmer for 20 minutes. Arrange the fish in the sauce. Heat the cognac gently, light with a match when it is hot and pour over the fish and sauce. Baste until the flames have died out and continue to simmer until the fish is done, only a few more minutes for haddock, bass and blue, but longer for monkfish. Remove pan from the heat and allow it to come to room temperature. Just before serving, line a platter with shredded endive, arrange the fish on it, nap with the sauce and garnish with lemon wedges, black olives and pepperoncini.

SUMMER FISH PATE (8 to 10 servings) 1 1/2 pounds firm white fish fillets 1/2 cup dry white wine 1 bay leaf 1 small onion, sliced 1 large carrot 1/4 cup frozen green peas 1 cup soft bread crumbs 1 small onion, finely chopped 1/2 cup heavy cream 1/2 cup mayonnaise 2 tablespoons lemon juice 2 large eggs 1/8 teaspoon cayenne 1/2 teaspoon salt Soft lettuce leaves such as bibb, black olives, cherry tomatoes and additional mayonnaise for garnish

Butter the bottom of a 7- or 8-cup loaf pan, line with waxed paper and butter the paper. In a skillet with a tight cover, arrange the fish fillets, white wine, bay leaf and onion slices. Cover, bring to a boil, turn down the heat and simmer for 5 minutes, or until the fish flakes. Cooking time will depend on the thickness of the fish. In the meantime, cut carrot into 1/4-inch dice and cook in salted water until soft. Add peas and remove the pan from the heat. When peas have defrosted, drain vegetables and set aside. Remove fish to a dish with a slotted spatula and reduce the liquid to 1/2 its original quantity. When both broth and fish are at room temperature, place in the blender container or food processor with bread crumbs, onion, cream, mayonnaise, lemon juice, eggs and seasonings. You may have to do this in two batches. Spread 1/3 of the fish mixture in the prepared pan and arrange half the carrots and peas on top. Add another third of fish, the remaining vegetables and the remaining fish, distributing the two vegetables as evenly as possible. Place the loaf pan in a larger pan and fill this with enough boiling water to come half way up the side of the loaf pan. Bake in a 375-degree oven for 40 minutes, or until puffed and lightly brown. Cool to room temperature and then chill for 3 hours or overnight. To serve, remove pa te' from the pan and peel off the waxed paper. Slice thickly and serve immediately on lettuce leaves garnished with black olives and cherry tomatoes. Pass additional mayonnaise.