From America's Test Kitchen
Most grilled tuna steaks are either rare in the center with no char or have a great sear enveloping a dry, mealy interior. We wanted a thick layer of hot, grilled tuna with an intense smoky char wrapped around a cool, delicately flavored, tender, and moist center.
We began by selecting tuna steaks that were thick enough to stay on the grill long enough to achieve a decent crust without overcooking. Our initial test of cooking methods proved that using direct heat with a hot fire and getting the tuna on and off the grill as quickly as possible worked well. We started by covering the grill with aluminum foil while it preheated so it became ultra-hot for a clean release for the fish, which can often stick to the cooking grate. For the charred flavor we were after, we turned to an ingredient that can enhance browning—oil. Oil helps to distribute heat evenly over the surface of the fish, including those areas not actually touching the cooking grate, and it adds a little fat to lean tuna, which keeps the exterior from getting too dry and stringy. But oil alone didn’t infuse our fish with grill flavor. We discovered that to moisten the tuna’s flesh, the oil needs to penetrate the meat’s tiny muscle fibers. Instead, we turned to a vinaigrette. The dressing (and its oil) clung to the fish, moistening its exterior and solving the problem of dry flesh. To improve browning we added honey to our vinaigrette. The sugars caramelized quickly on the grill, helping deliver a perfectly browned crust on our tuna steaks.
We prefer our tuna served rare or medium- rare. If you like your fish cooked medium, observe the timing for medium-rare, then tent the steaks loosely with foil for 5 minutes before serving. To achieve a nicely grilled exterior and a rare center, it is important to use fish steaks that are at least 1 inch thick.