THE INDEX finger plunges into the gills, flicks around, reappears, pushes on the tummy, is delicately inserted into the bed of crushed ice and rinsed clean. To us Westerners -- Lady Mary Henderson, wife of the British ambassador; Harry Simpson, her chef, and myself -- the sea trout are beautiful: red gilled, bright eyed, slippery skinned, good smelling.But the tall, young Japanese in white shirt and ducks tells Mitsuko Okawara, wife of Japanese Ambassador Yoshio Odawara that, the guts are not firm enough. The fish is too long dead for sashimi, for eating raw.

We are at the Baltimore Wholesale Fish Market on a Friday morning, on a food shopping expedition organized by Mary Henderson. Having had to learn on her own where to find quality food when she and Sir Nicholas came to Washington less than a year ago, she has been sharing her considerable knowledge with her newly arrived Japanese counterpart. Mrs. Okawara's chef speaks no English, so except when the embassy buys from the Mikado grocery on Wisconsin Avenue, Mrs. Okawara supervise the shopping herself.

The Japanese contingent have found a stall on the other side of the market with littleneck clams. The chef clanks them together, is satisfied with what he has learned of their inner life and buys. But he is really excited by the shrimp, large, fat, fresh shrimp at a good price ($8 a pound.) They buy crates of shrimp -- we use so much for tempura, Mrs. Okawara says -- which are loaded into the trunk of the car, black, official, with low-digit diplomatic plates. The clams go into the Styrofoam cooler and rubber boots are put up front to make room. (We had been told to expect a sloshy market. It wasn't.)

We Westerners decide to override the veto on the trout. After all, we will cook ours and they are lovely. Harry Simpson buys three MEDs (for medium, not, as we first thought, Mediterranean) weighing under two pounds each; along with a pound of backfin crabmeat. I buy two MEDs plus scallops and a few large shrimps. My fish are gutted (no extra charge), heads left on, wrapped in newspaper (shades of childhood) and placed in a nice plastic bag. Harry Simpson will clean his own.

At the Lexington Market, it is Harry Simpson's turn to become the authority. Mrs. Okawara learns about American cuts of meat and translates Simpson's expertise for her chef. The Japanese Embassy butter takes out his notebook and writes down the British Embassy's source for meat (District Hotel Service, E Street, SW). Everybody has learned from everybody.

The Lexington Market has a greater variety of fish and, according to the Japanese chef, better quality than the Wholesale Fish Market. But it's more expensive. Mary Henderson still cannot find the smoked cod's roe she prefers for her taramasalata, not even at the counter that sells possums and raccoons. We buy string beans, young and slight, for dinner and luscious oversized, red-black cherries for husbands.

Safely back in Washington and cooked, the sea trout were absolutely delicious. Here is how they were done, with scallop mousseline stuffing. SEA TROUT STUFFED WITH SCALLOP MOUSSELINE (8 servings) 2 whole sea trout (about 2 pounds each), gutted, with head left on 1 cup white wine 1 peled and sliced carrot 2 onions, pealed and sliced For the mousseline: 1/2 pound very fresh sea scallops 1 egg white Pinch or 2 grated nutmeg Salt and pepper to taste 2 tablespoons cognac 1 cup heavy cream For the garnish: 1/2 pound large shrimp, peeled but with tails on

Make the mousseline first. All ingredients should be very cold, including food processor bowl and blade (or blender, although the processor is easier) which should be refrigeratef for an hour or two before starting. Place scallops in bowl and process until smooth. With motor running, and egg white, seasonings, cognac and cream. Mixture should be the consistency of stiff whipped cream.

Wash and dry fish and fill with the mixture. Sew closed with trussing twine. Arrange the fish in a large roasting pan, seam side down, and pour the white wine into the pan. Add the carrot and onion, cover with foil and bake for about 30 minutes in a 425-degree oven. Remove foil and continue baking for an additional 10 minutes or so. Then add the shrimps to the pan for another 5 minutes. The fish is done when it flakes and the mousseline is firm.

A beurre blanc would be nice lilygilding, but really not necessary.

The string beans were cooked in a pot of boiling water until firm but not too undercooked and then mixed with a little hazelnut oil and a few drops of lemon juice.