On one of spring's lovliest evenings a Washington social rite - an outdoor reception - was in progress at the residence of the ambassador of Finland. Guests tasted Finnish vodka and wandered down the sloping lawn to examine the handsome cabin, housing an elaborate sauna beside a pool. At frequent intervals women in rutsic Finnish garb would move from group to group with trays of hors d'oeuvre.

They offered marinated salmon topped with dill sprigs, mushrooms with cream sauce in pastry shells, meat-filled turnovers, toast squares with puffed cheese topping. In the world of official cocktail parties ostentation and papier mache canapes are the norm. Here nothing was elaborate and the food had been given an artist's touch: presented in small and convenient sizes, graceful in design and distinctive to taste.

The architect of the presentation was Johanna Tuominen, whose husband, Leo, has been his country's ambassador to the United States for the past six years. Understatement and a passionate commitment to flavor and freshness in food are central to her philosophy of entertaining and eating.

"Food should always be as fresh as possible if a meal is going to be a success," she said during an interview several days after the party. "We heard in Europe that Americans merely open tins and eat. It may be easier, but we think it's nice if you make meals from scratch, that it shows you care and it gives you a feeling of accomplishment."

Madame Tuominen has a young chef in the kitchen to execute her menus, but she does the planning, uses some of her personal recipes and usually does the marketing herlsef. "I'm very particular," she said.

"We don't use strong spices so much in Finnish cooking," she said. "We belive the natural flavor of good ingredients should come through." Thus she does not follow the Swedish custom of serving a sweet, pungent mustard sauce with cured salmon. (As a canape, the salmon is topped with dill; at a luncheon there may be boiled new potatoes as well as dill beside the salmon.) Nor can she comprehend the American ritual of buying a fast food hamburger "and then adding prefabricated catsup to give it flavor. I don't think that's so good," she said. "I'm against chemicals. I know they have to use them, but they shouldn't use too much. Isn't it strange that when they use so much in the soul, they get large vegetables but not much taste is left?"

Madame Tuominen, a diplomat's wife after all, hastened to point out how much she enjoyed the year-round supply of fresh vegetables and salad ingredients available here and voiced her approval of American shellfish, fresh fruits and meat, though she added. "We like the lean (meat.) We never use the fat. We cut it off."

Her own style of entertaining is governed by some limitation. The residence dining room seats no more than 20 and "as a small country" Finland hasn't the resources to sponsor massive gatherings. "But when I serve, I try to do something from Finland," she said. "After all food is part of our culture, our life."

This led to a discussion of morels. The Finns are blessed with virgin forests and a plentitude of wild mushrooms. What they lack so far is an industry to export them. Madame Tuominen's shopper's eye had noted the price of imported mushrooms at the French market and without doubt that information has filtered back to Finland. Other native foods she treasures are cloudberries, which she will serve on a parfait whenever possible, baby shrimps (she gets them sometimes from Sweden, which is close enough) and bread. "I've never had real rye bread here, or at least not what we call rye bread," she said sadly.

Finnish cooking has much in common with Swedish (Finland was part of Sweden until the early 1800s) and has been influenced by Russian. Fish, Dairy products prok and sweets are popular - perhaps too popular for ideal nutrition - while there is an understandable shortage of green vegetables and fresh fruit during much of the year. A nation-wide effort to alter dietary habits and thereby lower the national blood pressure and reduce the high level of heart disease is paying off, Madame Tuominen reported.

"We are slowly beginning to eat frozen green vegetables in winter," she said. "We have freezers now, but I was reading that long ago in the Asian part of Russia and in Tibet they put food outside in winter to freeze. We have only reinvented this habit." She laughed. "It is a wide field once you start talking about food with me."

Here are several recipes from her collection. SALMON CURED WITH DILL

(About 100 canapes) 2 boned salmon filets with skin on, each about 2 pounds 2 tablespoons coarse salt 1/4 to 1/2 tablespoon sugar 1 tablespoon crushed black pepper 2 bunches fresh dill, trimmed of large stems

Mix together the salt, sugar and pepper. Spread most of the mixture over both filets. Place one filet, skin side down, stop a bed of salt and dill on a large sheet of aluminum foil. Spread most of the remaining dill over the filet, then make a sandwich by placing the second filet atop with skin side up. Spread remaining mixture and dill on top and fold foll over and in from ends to make a tight envelope. Refrigerate for 2 or 3 days, turning the envelope at lease once a day.

Remove filets from foil, wipe them clean with paper towels. Rewrap filets separately and place in freezer. Like beef, salmon slices more evenly when it is chilled and firm. They may be kept for 1 to 2 months. To serve, make diagonal slices as thin as possible across the grain, cutting to the skin but not through it. Cut slices into smaller pieces for canapes and place on thin-sliced bread or serve as is for luncheon or a dinner first course. Top each canape with a sprig of dill. Offer lemon and a pepper mill with sliced salmon. CHEESE SQUARES

(About 200 canapes) 2 pounds Swiss Emmenthal cheese 1 pound French Gruyere cheese 1 pound Canadian cheddar 4 sticks (1 pound) lightly salted butter, softened 5 egg yolks 1 tablespoon Dijon-style mustard 1 teaspoon paprika (mild Hungarian preferred) 1/2 cup cognae 2 loaves thin-sliced sandwich bread

Grate the cheeses finely. Place softened butter in a large bowl and mix in the egg yolks, mustard, paprika and cognac. Add cheeses and fold until an even-textured paste is obtained. (This may be stored in the refrigerator for up to a week. Remove it in the morning of the day it is to be used, however, so it will soften and be spreadable.

Spread a liberal amount ona piece of bread. Trim crusts and cut into quarters. Fill a baking sheet and store it in the refrigerator or another cool place until ready to bake. Place sheet in preheated 450-degree oven and cook until puffed and golden, 10 to 15 minutes. Serve hot COCKTAIL MEAT PASTRIES

(Makes about 32) 1 1/2 sticks (12 tablespoons) lightly salted butter 2 cups all-purpose flour 1/2 cup sour cream 1 tablespoon butter or oil 1 pound lean ground beef 1 medium onion, minced Salt and pepper to taste 1/2 cup seedless raisins, soaked briefly in hot water 2 egg whites and 2 whole eggs

Work butter, flour and sour cream together into a soft dough. Refrigerate while preparing filling. Melt butter in a frying pan. Brown meat, then lower heat and add onions. Cook, then season with salt and pepper and stir in drained raisins. Let cool.

Roll out dough to 1/4-inch thickness. Cut circles with a thin-lipped, floured glass, about 3 inches in diameter. Place about 1 tablespoon filling on each circle. Paint edges with beaten egg white and fold dough over meat to make a half-moon. Crimp edges together with fork tines. Place on baking sheets and chill until ready to bake. Preheat oven to 450 degrees. Paint the top of each pastry with beaten whole egg and make 3 small holes to allow steam to escape during baking. Cook until th pastries have risen and are nicely brown, 15 to 18 minutes. Serve warps. SALMON AND POTATO CASSEROLE

(Serves 6) 2 pounds potatoes (not Idaho baking), peeled and sliced 6 ounces cured salmon (commercial lox would be too salty), cut in strips 2 tablespoons mixed chives and dill 2 eggs 2 cups milk Dash of pepper 1 to 2 tablespoons breadcrumbs Butter

Grease a heatproof 1 1/3-or 2-quart casserole.Layer half the potatoes in the casserole. Cover with the salmon strips and sprinkle on the herbs. Cover with remaining potatoes. Beat eggs and add milk and pepper. Pour into casserole. Top with breadcrumbs and dot with butter. Bake, uncovered, in a preheated 350-degree oven until potatoes test done, 45 to 60 minutes.