Scale the fish by placing it in the sink under cold running water. Grasp the fish firmly by the gills and scrape off scales with a fish scaler or small, dull knife. Using short strokes, work from the tail to the head.
To remove the head, cut the flesh on both sides with a knife. If the fish is small, slice directly through the spine. For a larger fish, place the knife between vertebrae and tap the back of the knife with a hammer.
To fillet, use a sharp, thin knife. With the cod lying on its side, insert the knife behind the gills, and cut in an arc down to just above the backbone. Continue cutting parallel to the backbone toward the tail. Bring the knife up at the tail and remove the fillet.
To make steaks, place the cod on its side. With a sharp knife, cut slices 3/4 to 1/2-inch (1.25 to 1.9cm) thick perpendicular to the spine. Tap the back of the blade with a wooden mallet to cut through the spine.
The secret to successful cod cookery is to not overcook. Whichever of the following cooking methods you choose, your cod will be cooked when the flesh becomes opaque and is easily pierced with a fork.
Rinse the fish and pat dry with a paper towel. Place the whole fish, fillets, or steaks in a baking pan. Brush with butter or oil or cover with sauce made of liquid, herbs, spices, and vegetables. Bake in the oven at 400°F (200°C) until a knife slice in the thickest part reveals the flesh to be opaque but still moist.
Place fillets or steaks on perforated aluminum foil on the grill, 4 to 6 inches (10 to 15cm) above prepared coals or fire. Baste with butter, oil, or marinade, and close the hood of grill. Cook until fish is opaque and moist on the inside, about 6 to 8 minutes for fish less than 1-inch (2.5cm) thick, and 10 to 15 minutes for fish larger than 1-inch (2.5cm) thick.
Rinse cod, and pat dry with a paper towel. Dredge in flour and seasonings. Shake off excess flour. Heat the frying pan until hot, and add butter or oil. Place the fillets in the pan and cook, turning once, until fish is opaque but still moist in the center, about 2 to 10 minutes, depending on the size of the pieces.
Pour at least 1 1/2 inches (3.8cm) of oil into a wok or deep fryer; the cooker should be less than half full of oil. Heat oil to 375°F (190°C), using a thermometer to monitor temperature. Cut cod into similar sized pieces, 1 1/4 to 1 1/2-inch (3. to 3.8cm) thick. Dip in batter, drain, and then slip pieces into the hot oil. Cook until brown, about 2 to 3 minutes.
Bring poaching liquid, consisting of water, broth, herbs, and spices, to a simmer. Slip in skinless fillets, then cover pan and keep poaching liquid at a simmer for about 8 minutes per inch (2.5cm) of thickness.
Rinse cod fillets or steaks and pat dry with a paper towel. Coat fish with flour, crumbs, or cornmeal if desired. 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. Broil, turning once, until the fish is opaque but still moist in the center—about 3 to 10 minutes, depending on size of the fish.