So many foods are becomming a once-a-year treat, a reward for being so good with lentils and sauteed liver the rest of the days: standing rib roasts of beef, and poached salmon with tarragon mayonnaise, and lobster any way at all.

Lobster is a common and costly sight all over the world. Possibly every single one in our cold northern waters could find a home on some foreign dinner plate. And if you think the price of Maine lobster is high in the United States, can you begin to guess what it is in Paris or London?

For years the industry has been trying to convince its customers that the best way to buy lobsters is by the pound of picked meat. Most of the time, that pound is made up of the tails and claws of five or six lobsters, and is a much better buy than those au naturel.

Most people familiar with lobsters will hesitate if you offer them a big one -- 10 pounds or so; the meat, they say, is tough.

"It's just one of those lobster myths," says James Fair, assistant director of Maine Fisheries. "The meat is just as tender and succulent as the little ones."

Should you travel down the ocean road in New England, stopping off at every lobster pound along the way, you will find that opinions on how to cook them differ fiercely.

Most cookbooks talk about large pots of boiling water. But at one busy pound, the wife of the owner held her thumb and first finger two inches apart when asked how much water she puts in the pot. "Think of all the good flavor you boil away," she said.

Splitting and broiling is not done. At one market in New Hampshire, a young woman looked horrified at the mere request, and downright refused to do it. "That's a terrible thing to do to a lobster," she said angrily.

Anyone who can row a boat, afford a license and lay hands on a few lobster traps is in the business. So why are they so expensive?

Lobstermen who once put out 200 traps are now using 600. So are there fewer lobsters, or just more traps?

Truly, there are fewer lobsters. One local fisherman talked wistfully about family picnics 30 years ago. The adults would build the fire, and the kids in the family were sent out to scrounge around the shore for lobsters. Evidently the lobsters now feel pursued, and have moved out to deeper water.

Like putting a teaspoon of caviar into a Russian salad dressing to give six people an expensive taste treat, it is possible for one lobster to be shared by four. Here is one way to get around the high prices, and it allows more people to enjoy this particularly rich and distinctive shellfish at more than one summer picnic. LINGUINE WITH CREAMY LOBSTER SAUCE (4 servings) 1 or 2 lobsters, 1 to 1 1/4 pounds apiece* 2 cups water 3 tablespoons butter 1 tablespoon olive oil 1 carrot, chopped 1 medium onion, chopped 1 stalk celery, including leaves, chopped 2 cloves garlic, minced 1/2 teaspoon rosemary 1 lemon, juice and grated rind 2 tablespoons tomato paste 1/8 teaspoon cayenne pepper 1/4 cup heavy cream 12 ounces linguine 1/4 cup minced parsley

Steam lobsters on a rack over boiling water, 10 minutes for 1 pound and 14 minutes for 1 1/4. When cool enough, pick out all the meat and save shells.

After removing meat from legs and chest section, drop shell and cartilage into a blender container or food processor. Add 2 cups water and blend for 1 minute. Combine in saucepan with other shells, bring to a boil, and simmer gently 15 minutes, or until liquid has reduced to 1 1/2 cups.

Heat butter and oil in a skillet and saute' carrot, onion and celery about 10 minutes, until soft, but not brown. Add rosemary and 1 clove of minced garlic and simmer 1 minute.

When lobster broth is done, pour through a strainer lined with a wet paper towel into skillet with vegetables. Add lemon juice and whisk in tomato paste and cayenne, bring to a boil, and simmer briskly until sauce thickens, about 15 minutes. Remove from heat; cool.

Pour contents into blender container or food processor and pure'e. Return to skillet and whisk in heavy cream. Simmer gently for 5 minutes.

Chop lobster meat into 1/4-inch cubes and add to sauce. Mince together remaining garlic, grated lemon rind and parsley.

Cook linguine in plenty of boiling, salted water until just al dente. Drain. Pour a little sauce into bottom of warm platter; add linguine and pour remaining sauce over it. Sprinkle with lemon rind and parsley mixture. Toss just before serving to four.

*Note: Even with one 1-pound lobster, the sauce will have a good flavor. The more, though, the better.