THE BUBBLES continued their relentless escape from the two-hour-old glass of sparkling water. The violin and piano rose to an unexpected crescendo, rousing one luncheon guest just in time to prevent his chin from falling on his chest.

It was 3 o'clock on a warm and brillant May afternoon. Chauffeurs lolled on the grass beside long black limousines lining the driveway of the Finnish Embassy. In the dining room of the enormous stone house, 30 guests were listening to a musical interlude following the main course of pate-stuffed reindeer tongues with cranberry sauce. The dessert of cloudberry mousse had yet to be served.

The lunch was preceded by an hour-long tasting of Inglenook wines beginning at noon.There were six more wines at lunch; by mid-afternoon five of them had been served. If the minds of a few guests wandered, they might be forgiven their reveries. But there was nothing somnolent about the food, and the attentiveness of the Ambassador and Mrs. Jaakko Ilomieni never wavered.

Finnish cuisine is hardly a familiar concept in America. When one thinks of it, if at all, what comes to mind is fish stew or other hearty food to relieve the chill of long hard winters.

A typical remark was made by a member of Vice President Mondale's party after they had traveled through Scandinavia: "Eleven nights in a row-fish and potatoes."

But Finnish food is far more imaginative than that, as the exotic meal prepared by Helena Ilomieni and her cook Merja Nuutinen proved.

Mrs. Ilomieni and the cook had indeed served fish for the six-course lunch-"two of those large very expensive salmons," as Mrs. Ilomieni described them. The fillets had been poached in a court bouillon, flavored with bunches of fresh dill, then cut into small cubes and served in a clear broth with more fresh dill. There were potatoes, too, but they were incidental to the reindeer tongue, which makes beef tongue taste like a pale, tough copy in comparison.

The star attraction, however, was the morel tarte, with the woodsy fragrance and flavor of wild mushrooms, some of which grow as big as melons in the Finnish countryside. Mrs. Ilomieni had carried enough dried morels back from her trip to Finland to make four tartes. The nearest Americans can get to Finnish dried morels are those imported from France at $11 an ounce-or on a tarte by tarte basis, $90 each just for the morels. "We only have one or two pies in the summer when we are home," Helena Ilomieni said. "They are not ordinary food."

And they are difficult to find in April. The Ambassador had to call upon a restaurateur friend in Helsinki to come up with enough for the lunch.

Helena Ilomieni also brought back the cloudberries, which look like yellow raspberries, the frozen reindeer tongues, five Finnish cheeses and some handmade chocolates. She is the dedicated chatelaine of an embassy whose guests are always served "homemade" food prepared in a small, 1930s kitchen. "Never! " she said when asked about catered meals. "I must be almost half dying upstairs before I would allow it."

The soft-spoken woman who apologizes because "English is my fourth language" cares deeply about food because her mother and grandmother before her did. "My mother was a great cook and my grandmother was a marvelous cook. She was like a catering cook in the village where she lived."

Mrs. Ilomieni had gone to an enormous amount of trouble for the afternoon-long lunch honoring the 100th anniversary of the Inglenook vineyard, which was founded by Finnish sea captain Gustave Niebaum. "We only entertain for commercial things if there is a little art mixed in," the ambassador explained. "We had such a wonderful time at Inglenook, we wanted to do something for them," Helena Ilomieni said.'

It was something. A commercial enterprise looking for a suitably elegant setting to entertain could not have found better, at any price, in this city. The imposing 60-year-old stone residence sits on top of a hill near the Shoreham Hotel surrounded by homes where most of the dining rooms can seat 30 with ease. Trees as old as the house shade it completely from the sun and the street. The sloping back lawn leads to a pool and house with sauna.

The high-ceilinged building, decorated in Finnish modern and period French, looks deceptively flawless. But it must be torn apart this fall and redone. The Ilomienis will move out for 18 months so that a new kitchen can be installed and two rooms added, along with new plumbing and wiring and a new swimming pool. The Finnish government would like to tear down the old house and build one in Finnish-modern style, but the neighbors won't have it. Washingtonians "are not very hospitable to embassies," the ambassador said. "You think of us as people who park in the wrong places," he added with a smile.

Nothing was out of place during lunch. The food was properly hot, though it had to be sandwiched in between welcoming remarks, wine tasting remarks, the opening of a surprise bottle of wine from 1891 (which had failed to survive the 88-year journey) and the concluding toast.

An hour after the sleeping guest was roused by the concluding bars of Brahms, the guests withdrew to the living room for coffee. When the harpist-who began playing at noon and had stopped only for the speeches and the musical interlude-carried her harp out the front door at 4:20, the party was still going on.

These are three of the recipes served at the lunch. German dried mushrooms have been substituted for the morels. Instead of $11 an ounce, they cost $6.

WILD MUSHROOM TARTE

(12 first course servings)

Crust:

2 cups unbleached flour

1/2 pound butter or margarine, softened

1/2 teaspoon salt

4 or 5 tablespoons cold water

Filling:

6 to 8 ounces dried mushrooms*

2 tablespoons butter

1 to 1 1/4 cups heavy cream

Salt and freshly ground black pepper to taste

6 shallots, finely chopped

2 egg yolks, slightly beaten

1/4 cup sherry(FOOTNOTE)

* If the mushrooms you purchase are finely chopped, buy one ounce of whole dried mushrooms and proceed with the soaking. Then cut them up coarsely and mix them in with the finely chopped mushrooms. This gives a lighter and better look to the tarte. (END FOOT)

The day before the tarte is to be made, cover the mushrooms with cold water and soak, changing the water 1 or 2 times, depending on the gritness of the mushrooms.

To make the crust, cut the butter into the flour and salt. Add enough water to make very soft dough. Chill the dough for one hour. Place the ball of dough in the bottom of a 12 inch flan ring. Using your fingers, flatten it to conform to the bottom of the pan and then shape it to fit the fluted sides of the ring. Refrigerate for at least an hour, overnight if desired.

To make the filling, drain the mushrooms and chop. Saute them in 2 tablespoons butter for 5 minutes, until tender. Add the shallots, salt and pepper to taste and saute until shallots are soft. Add the sherry and enough cream to moisten the mushrooms but keep the mixture thick. Cool to room temperature. Stir in the yolks and pour mixture into crust. Bake at 375 degrees for 50 to 60 minutes, until the crust is golden and filling is firm. Serve warm not hot.

LAPLAND SALMON SOUP

(15 servings)

8 to 10 pound whole salmon

3 1/2 quarts water

2 carrots

2 leeks

1 medium onion, quartered

1/2 large celery root

1 1/2 small bunches dill

10 white peppercorns

10 black peppercorns

1 tablespoon thyme

5 bay leaves

salt to taste

8 egg whites

2 cups chopped onion

fresh chopped dill

Remove the head and tail of the fish and fillet it, leaving on the skin (or have the fishmonger do it for you). Combine the head, tail, bones and a little of the flesh along with the water, carrots, leeks, onion, celery root, dill, white and black peppercorns, thyme, bay leaves and salt in a large pot and simmer for 25 to 30 minutes. Strain and cool. This is the court bouillon. Beat the whites slightly and add to the court bouillon. Bring to a boil. (The whites clear the stock of any impurities). Place a cheesecloth in a strainer and pour the bouillon through the cheesecloth.

Cut each salmon fillets into 3 pieces and poach in just enough court bouillon to cover the fish. Cover the pan and cook, just below a simmer for 5 minutes. Remove the fish and when it is cool enough to handle, remove the skin. Cut the fish in small pieces.

Heat remaining bouillon and when it is quite hot, but not boiling, add the chopped onion and the salmon pieces. Cook 3 to 4 minutes and serve immediately, either in individual bowls or from a tureen. Sprinkle each serving with freshly chopped dill.

OLD FASHIONED EGG DRESSING

(6 servings)

2 hard-cooked eggs

2 teaspoons Dijon mustard

1/2 cup sour cream*

2 teaspoons white wine vinegar

Salt and white pepper to taste(FOOTNOTE)

* Other possibilities are plain yogurt or buttermilk. (END FOOT)

Combine crumbled egg yolks with remaining ingredients and serve over sald of cucumbers slices and greens. Top with chopped egg white. CAPTION: Picture 1, Aili Heinonen of the Finnish Embassy; Picture 2, Helena Ilomieni with Bob Furek, general manager of Inglenook; Picture 3, Mrs. Ilomieni with cook Merja Nuutinen, by Harry Naltchayan, Margaret Thomas-The Washington Post.