WHEN YOU deal with Germaine, you're always a guinea pig," said a guest savoring one of her "experiments" before a home-cooked dinner.

Dick and Germaine Swanson were used to having friends stop by their Bethesda ranch-style home unannounced, just for a drink. But in the days before she opened the Pan-Asian restaurant which bears her name, it was impossible to get away with just a drink.

"Germaine is like a Jewish grandmother," said the guest, photographer Stanley Tretick. "My grandmother used to say "Eat, eat, eat! Germaine does essentially the same thing-with a lot more finesse."

Germaine, tossing a Caesar's salad at the inlaid oak-and-teak table top which divides the kitchen from the solarium, acknowledged the syndrome: "For Asians, if you eat a lot, it means the food is good. If you come in we put an extra bowl. We are very happy-we don't think you are taking any food away."

Germaine's home-cooked dinners are rare since the restaurant opened last September. Usually her mother (who lives with the Swansons and works in the restaurant during the day) cooks or brings home food from the restaurant for the children: Justin, 7, and Phillip, 16. Once or twice a month they eat out or Germaine does the cooking. For the boys, the real treat is having their mother do the cooking in their own kitchen. It's usually Vietnamese food, or American food with a Vietnamese twist-though Justin says he likes hot dogs best. The family always uses chopsticks, as do guests, except on rare occasions.

One occurred recently when grilled steak was the entree, and there were three guests for dinner. The meat had been tenderized in a Vietnamese marinade. Dick cooked it on the outdoor barbecue and poured the wine. Germaine did everything else: prepared the marinade, gazpacho, salad and dessert; set the table; and washed the dishes Dick Swanson, sipping the last of his red wine as Germaine cleared the table, said she was doing so much only because "she doesn't want to ruin my masculine image. After all, I do the dishes, cook the meals, do the laundry, get the kids off to school . . ." The rest was lost in peals of laughter.

The steak was made from beef knuckle, ("Ask for sirloin tip. It's much cheaper and better than filet") the same cut used in the restaurant for the grilled Indonesian beef sates and stirfried dishes containing beef. The meat had been marinated overnight. Germaine says the secret of tender meat is to take it out of the refrigerator two or three hours before charcoaling.

The gazpacho was made with V-8 juice and canned beef consomme, but she joked about admitting that she had used canned soup: "They'll think I use cans in the restaurant." But, of course, she doesn't. She has 15 people working there and fresh beef stock is always available.

The vegetables for the cold soup however, were chopped by hand. Germaine would never use the food processor: It makes the soup "cloudy."

She will make a few compromises to save time. The croutons for the salad came from a box, but the nitrite-free bacon was purchased at the Farm Woman's Market, although it meant an extra trip. And the whole pineapple served for dessert was carefully peeled and then scored because "making it look nice is part of Asian cooking."

But before any of that was served, everyone was expected to sample a slice of boneless chicken roll which had been stuffed with vegetables, eggs and ground chicken and covered with Indian sauce-a mixture of coriander, turmeric, cumin, onion and ginger. And they were expected to offer an opinion. If Germaine's latest creation is well accepted around her kitchen table, it often appears on the restaurant menu the next day.

It was and it did.

No matter what kind of food Germaine is cooking, her guests are always part of the process, sitting at the table (strips of wood nailed together and attached to one wall) or standing around as Germaine does some of her lightning-swift chopping on the wooden top. Without walls between the living room, kitchen and solarium-entrance, the open space makes everyone feel welcome to kibbitz and drink wine, or whatever else they like while Germaine cooks.

The house wasn't always like that. When the Swansons bought it in 1972 there were six cell-like bedrooms because the previous owners had three sets of twins. Besides tearing down most of the inside walls, the Swansons redid the kitchen, putting in wooden countertops and cabinets. The white six-burner stove-the only piece of equipment which distinguishes Germaine's kitchen from thousands of others in the suburbs-is her pride and joy. But it took her an entire year to remove the accumulated grease and grime.

Around the house, there are many Oriental touches: the small family altar on the wall above the kitchen table, the brasses and prints, and especially the haunting color photographs of Vietnamese families and scenery taken by Dick Swanson.

Dick and Germaine met in South Vietnam in 1965 when she was a reporter for Time and Life magazines and he was a photographer for Life. They were married there in 1969 and came to Washington in 1971.

Dick brought 12 members of Germaine's family out of South Vietnam on the last United States government flight to leave the country before it fell in 1975. They were used to being refugees. Several years before, the family had fled North Vietnam, where Germaine's father had owned a French restaurant and night club.

Germaine did not learn to cook in the restaurant. Instead, she washed glasses, for which her father usually gave her five pennies. What she knew about cooking when she came to Washington she remembered from her grandmother: "Grandmother was a very good cook and she used to tell the children all the time about all the things she made. I told her she talked too much, but I guess I had all those things in my brain, because I knew what to do when I started here."

Germaine began experimenting here because she didn't like the Oriental food served in restaurants. "It didn't have spices, they took short cuts and the MSG made me sick. So I decided to cook at home for Dick and his friends. I didn't have any confidence, but my friends kept telling me how good it was and wanted me to teach them how to cook."

Germaine started her classes with eight students. A newspaper article brought 300 more and she taught in her home for two years. When her family arrived, she began to think about opening a restaurant to employ them and spent several years learning everything from the books to the front of the house by working in several area restaurants.

At first she spent most of her waking hours at the restaurant because she was frightened about paying the debts. "The first three months were very tough for me. I was shaking like crazy. I didn't know what was going on. Dick and I would look at each other and never talk about it, but we were worried."

But business is so good now that Germaine and Dick can laugh about the difficulties of borrowing money to open the restaurant at 2400 Wisconsin Avenue. "I mortgaged Dick to the bank. Dick mortgaged my mother and we mortgaged the children," she jokes.

The success of the restaurant hasn't given Germaine any more free time, just a little peace of mind.

These are recipes for Germainehs dinner at home.

This recipe should serve 12 or more, but there was little left over after 7 people kept eating, and eating and eating!

GRILLED STEAK IN VIETNAMESE MARINADE 6 pounds of sirloin tip steak, each steak 3 or 4 inches thick 1/2 cup Japanese soy sauce 1/3 cup Maggi sauce 1/3 cup salad oil 1 small onion, chopped 4 cloves garlic, chopped Black pepper to taste

Score the surface of the steaks, making diamond pattern so that the marinade will soak in and when the flame gets into the crevices, it will be crisper. Combine all of the remaining ingredients and place steaks in marinade in the refrigerator, either ovenight or for at least four hours. Grill steak over charcoal. Swanson does his in a covered grill.

GAZPACHO

(5 servings) 1 can beef consomme of 10 1/2 ounces beef stock Equivalent amount of V-8 juice 1 green pepper, finely chopped 3 medium tomatoes, seeded and finely chopped 1 cucumber (any size) peeled, seeded and finely chopped 1 tablespoon white vinegar 2 tablespoons salad oil Couple dashes Worcestershire sauce Dash hot pepper sauce Couple dashes white or black pepper

Combine liquid ingredients and stir in vegetables. Chill quickly.

CAESAR'S SALAD

(6 servings) 1 head romaine lettuce 1/3 cup salad oil 2 cloves garlic, minced 2 egg yolks Dash hot pepper sauce Couple dashes Worcestershire sauce White pepper and salt to taste Lemon juice Croutons Nitrite-free bacon

Break up lettuce. Combine oil with garlic. Beat in eggs until mixture is smooth. Beat in hot pepper sauce and Worcestershire. Season with salt and pepper. Pour over lettuce and mix. Squeeze juice of half a lemon on top. Sprinkle on croutons and bacon and serve. CAPTION: Picture 1, Germaine Swanson's home-cooked dinners are rare since her restaurant opened last fall.; Pictures 2 and 3, It's a treat when her family has her all to themselves as they did recently when Germaine and husband Dick, left, prepared dinner for sons Justin and Phillip and photographer friend Stanley Tretick, right. Photos by Tom Allen-The Washington Post. Picture 4, Geramine Swanson with sons Justin and Phillip, by Tom Allen-The Washington Post