In general, the cooking time for fish is 10 minutes for every inch of thickness - whether you bake, poach, broil, or grill. To test for doneness, slip the point of a sharp knife into the thickest part of the fish and pull aside. If flakes begin to separate, the fish is probably done. Remove fish from heat and let it stand 3 to 4 minutes to finish cooking.

Baking
Rinse fish and pat dry with a paper towel. Whole fish may be stuffed with rice and vegetables. Place whole, boned fish in a baking pan. Brush with butter and oil and season with salt and pepper, or cover with a piquant sauce. Bake in a preheated oven at 400°F (200°C) until a knife slice in the thickest part reveals the flesh to be opaque but still moist.

Grilling
Place whole small fish or fillets on perforated aluminum foil over a greased grill, 4 to 6 inches (10 to 15cm) above prepared coals or fire. Baste with butter, oil, or marinade, and close hood of grill. Cook until opaque and moist on the inside, 6 to 8 minutes for fish less than 1 inch (2.5cm) thick; 10 to 15 minutes for fish larger than 1-inch (2.5cm) thick.

Broiling
Rinse whole fish, fillets, or boned and butterflied fish and pat dry with a paper towel. Place fish on a rack above a baking dish. Preheat broiler and adjust oven rack so fish is 3 to 4 inches (7.6 to 10cm) from the element. Brush with butter or oil and season with salt and pepper. Broil, turning once, until fish is opaque but still moist in the center, 3 to 10 minutes, depending on size of the fish.

Frying
Rinse fish, and pat dry with a paper towel. Dredge in flour and seasonings if desired. Shake off any excess flour. Heat frying pan until hot, then add butter or oil. Put in fillets and cook, turning once, until fish is opaque but still moist in the center, 2 to 10 minutes, depending upon size of the fish.

Poaching
Bring poaching liquid, consisting of water, broth, and herbs and spices, to a simmer. Slip fish in, then cover pan and keep liquid at a simmer for about 8 minutes per inch (about 2.5cm) of thickness.

Steaming
Place fish on a greased perforated rack over 1 to 2 inches (about 2.5 to 5cm) of rapidly boiling water. Cover with a tight-fitting lid and keep water at a constant boil through cooking time, 8 to 10 minutes per inch (about 2.5cm) thickness of fish.