Cooked fish should be moist. To tell if a whole fish is cooked, insert a skewer near the backbone; it should meet little resistance. A fillet is done when the flesh is opaque at the thickest part; a steak barely flakes when it’s pricked with a fork. Allow 10 minutes of cooking time per inch of flesh thickness, no matter what method you use.
Baking is especially suitable for a whole fish. Use a generously buttered dish and brush the fish with butter or oil; or cover it with chopped vegetables or a mixture of bread crumbs, butter, and herbs. Bake in a preheated 450°F oven for the prescribed time.
Another baking method is to wrap fish in oiled foil, add seasonings or a sauce, and bake at 450°F Wrap the foil loosely to allow steam to circulate, but seal its edges tightly.
Frying and sauteing are the best cooking methods for small fish, such as smelts. First dip the fish in beaten egg, then coat with seasoned flour or bread crumbs; or just coat them with flour. To saute, heat equal amounts of oil and butter in a frying pan over moderate heat. Brown the fish on one side, then the other. To fry, heat oil to 375°F.
Broiling and grilling are suitable for any fish except those shorter than 6 inches. Preheat a broiler or have coals glowing in a grill. (A fish-shaped metal basket facilitates turning a whole fish on a grill.) Cook the fish 4 to 6 inches from the heat source, basting with oil or clarified butter.
Poaching (simmering in liquid) is best suited to firm fish, such as sole, salmon, red snapper, trout, or bass. To poach a large whole fish, leave the fins intact and support it on a rack in the poaching pan. Wrap small whole fish in cheesecloth for easy removal from the pan. Cover the fish with tepid liquid, bring to a simmer, then start timing the cooking. Court bouillon for fish In an enamel pan put 2 small whole onions, 1 carrot and 1 celery stalk cut up, 2 sprigs parsley, 2 sprigs thyme (or 1/2 teaspoon dry), 1 bay leaf, 1 teaspoon salt, 1 quart water, and 2 cups dry red or white wine or 1/2 cup wine vinegar. If you have some fish heads and bones, add these also. Simmer uncovered for 1/2 hour. Strain liquid through a double layer of cheesecloth.