From America's Test Kitchen
Moist and rare in the middle with a seared crust, pan-seared tuna is a popular entrée in restaurants. This dish is so simple that we thought it would be easy to make at home, and set out to determine the best method.
Starting with high-quality tuna—sushi grade if possible—is paramount; we prefer the flavor of yellowfin. A thickness of at least an inch is necessary for the center of the tuna to be rare while the exterior browns. Before searing the tuna in a nonstick skillet, we rubbed the steaks with oil, then coated them with cracked peppercorns; the oil helped the seeds stick to the fish. The peppercorns formed a beautiful, spicy crust. We learned that tuna, like beef, will continue to cook from residual heat when removed from the stove, so when the interior of the tuna was near the desired degree of doneness (about 110 degrees on an instant-read thermometer), we transferred it to a platter.
Most members of the test kitchen staff prefer their tuna steaks rare to medium-rare; the cooking times given in the recipe are for steaks cooked to these two degrees of doneness. For tuna steaks cooked medium, observe the timing for medium-rare, then tent the steaks with foil for 5 minutes before slicing. If you prefer tuna steaks cooked so rare that they are still cold in the center, try to purchase steaks that are 1 1/2 inches thick and cook them according to the timing below for rare steaks. Bear in mind, though, that the cooking times below are estimates; check for doneness by nicking the fish with a paring knife. To cook only two steaks, use half as many seeds, reduce the oil to 2 teaspoons on both the fish and in the pan, use a 10-inch nonstick skillet, and follow the same cooking times.