Marinating tuna steaks for 15 minutes to four hours over medium heat on a greased grill yields the best results. Marinating is not essential, and lightly brushing the tuna steaks with butter or olive oil just before grilling is sufficient to impart flavor and prevent sticking.
Tuna can become chewy and dry when overcooked, so it is advisable to stay near the grill during the cooking time. Tuna steaks are best grilled uncovered with direct heat for about four to six minutes per one-half inch of thickness. The fish should flake when tested with a fork and be pink in the center.
To provide caramelization, turn the steaks once halfway through cooking and brush with additional butter or olive oil.