Remove the strong-tasting brown streak running through the meat.
The secret to successful tuna cookery is to not overcook it; overcooking makes tuna dry out. Whichever of the following cooking methods you choose, your tuna will be cooked when its flesh becomes opaque yet is still moist on the inside.
Brush with melted butter or oil and season with salt and pepper, or cover with a piquant sauce. Place tuna in a greased baking dish, or wrap in oiled foil and place on a baking sheet. Bake in a preheated 450°F (230°C) oven 10 minutes per inch (2.5cm) of thickness of the fish.
Place tuna over hot coals on a well-oiled grill. Baste frequently and turn once halfway through the cooking period. Because tuna is lean compared to other protein-rich foods, it does not exude a lot of self-basting fat. Be sure all grills, baskets, racks, and foil are well-oiled to ensure easy handling of tuna while barbecuing.
Place seasoned tuna filets or steaks on a well-greased broiler pan and brush with oil. Broil under preheated broiler 4 to 5 inches (about 10 to 12.5cm) from heat until the flesh is opaque yet moist on the inside.
Heat a pan, add a small amount of hot butter or oil, and brown uncoated tuna steaks or fillets, turning once after 3 to 4 minutes and cook the other side 3 to 4 minutes.
Bring poaching liquid, consisting of water, broth, and herbs and spices, to a simmer. Slip tuna in, then cover pan and keep liquid at a simmer for about 8 minutes per inch (about 2.5cm) of thickness.
Place tuna on a greased perforated rack over 1 to 2 inches (about 2.5 to 5 cm) of rapidly boiling water. Cover with a tight-fitting lid and keep water at a constant boil through cooking time.