While grilling (holding a chunk of meat above the fire) might be the oldest form of cookery, poaching (putting a fish in simmering liquid) was probably not far behind.

For a simple dinner, a poached fillet of fish with a sauce made by reducing the cooking liquid is fast and easy. And for a big party, a large poached whole fish, such as a salmon, skinned and decorated with slices of cucumber to mimic the scales, is an impressive and delicious centerpiece.

Any firm-fleshed fish can be poached. For fillets, any of the imported fish such as Dover sole, turbot and John Dory are excellent, as are domestic petrale sole, catfish, orange roughy and whiting. For large fish, salmon and sea bass are good choices. Fish such as flounder, cod, red snapper and halibut have less solid textures and tend to fall apart when moved after being cooked. While you can poach a fish in salted water, it will be far more distinctive if cooked in a court bouillon, an infused liquid that enhances the fish flavor without overwhelming it.

The Basic Bubbles The idea of poaching, whether on top of the stove in a fish poacher or in a baking pan in the oven, is to cook the fish submerged in a liquid that is barely maintaining a simmer. It should be about 200 degrees with some small bubbling. It should not be at a rolling boil that will break the delicate fish fibers apart with agitation.

Fillets for poaching should be about 1/4 to 3/8 inch thick. Score the skin side lightly in a diamond pattern, if there is skin, and sprinkle the fillets with salt and pepper. After pouring the hot liquid over them, cover them with a sheet of buttered wax paper, and bake in a 350-degree oven for 5 to 7 minutes. Drain the pan juices into a saucepan, reduce them, flavor them, and have dinner on the table in a matter of minutes.

Whole fish are best wrapped in cheesecloth to keep them together when they are being maneuvered out of the poaching pan. While whole fish can be oven-poached, I think it's better to poach them on the stove, so you know exactly when to start timing and can readily adjust the intensity of the simmer.

Watching the Clock To time the poaching, measure the fish and allow 10 minutes for each inch of thickness; begin timing when the liquid starts to simmer. To check for doneness, insert a metal skewer into the thick flesh behind the gill. It should slide in easily. While small whole fish such as trout can be poached in almost anything, a six-pound salmon needs either a fish poacher or a roasting pan fitted with a rack at the bottom for easy lifting of the fish. I long ago turned my poacher into a planter and use the same roaster that holds turkeys on other occasions.

Since a roaster is not a long fish-like shape, it usually means the fish has to be curved to fit in. The benefit of this is that it retains this jaunty pose once cooked so it fits more easily onto a platter. Prior to poaching, allow whole fish to chill thoroughly before starting to skin and dress them. The flesh will be firmer, and less likely to break.

A Saucy Story Depending on what you've used as the poaching liquid, a sauce for the fish can be as easy as boiling down the liquid. Court bouillon and/or white wine should be reduced by at least two-thirds to reach sauce intensity. If you want a sauce, put very little salt in the poaching medium at the beginning, or the reduction will render the brew too salty.

After the liquid is reduced, whisk in anything you would like: chopped herbs, finely chopped tomato, a squirt of lemon juice. For richness, swirl in a tablespoon or so of butter or a few tablespoons of heavy cream. Then taste for seasoning and spoon the sauce over the poached fish.

COURT BOUILLON (Makes 1 quart)

1 quart water

1 onion, peeled and sliced

1 carrot, peeled, trimmed and sliced

2 celery stalks, trimmed and sliced

1 bay leaf

2 sprigs parsley

10 peppercorns

2 sprigs thyme

1/2 teaspoon salt

2 cups dry white wine

Bring the water to a boil with the onion, carrot, celery, bay leaf, parsley, peppercorn, thyme and salt. Simmer for 20 minutes, uncovered, add the wine, and simmer for 15 minutes more, uncovered. Strain before using.

Note: The court bouillon can be reused, straining it between uses, and it can be frozen for up to six months.

Per serving: 104 calories, .8 gm protein, 7 gm carbohydrates, .2 gm fat, 0 gm saturated fat, 0 mg cholesterol, 363 mg sodium.

FISH FILLETS WITH ASIAN VEGETABLES (6 servings)

2 pounds fish fillets

6 scallions, trimmed and sliced, including all of the green part

2 cups bok choy cabbage, sliced thinly on the diagonal

1/2 pound fresh snow peas, trimmed and cut into a fine julienne

1 cup court bouillon (recipe above)

1/4 cup soy sauce

1/4 cup sesame oil

1/4 teaspoon pepper

1 teaspoon minced garlic

2 teaspoons finely minced fresh ginger

Arrange the fish fillets in a baking dish. Arrange the scallions, bok choy and snow peas around the fish.

Bring the court bouillon to a simmer, along with the soy, sesame oil, pepper, garlic and ginger. Pour this mixture over the fish, cover fish with a sheet of buttered wax paper, buttered side down, and bake in a preheated 350-degree oven for 7 minutes. Remove from the oven, drain the cooking liquid while keeping fillets warm. Reduce cooking liquid by half. Serve immediately, dividing the fish among the plates along with the vegetables and pouring some sauce over each.

Per serving: 323 calories, 43 gm protein, 7 gm carbohydrates, 12 gm fat, 2 gm saturated fat, 71 mg cholesterol, 841 mg sodium.

-- Ellen Brown is a Washington-based food writer and prize-winning author of "The Gourmet Gazelle Cookbook."