Tuna is one of the fatty fish rich in omega-3 fatty acids that the American Heart Association suggests eating twice or more weekly. However, because tuna runs relatively high in mercury, have it only as an occasional part of your seafood rotation, along with salmon, mackerel, cod, trout and other fatty fish. Ahi tuna steaks cook quite quickly; too quickly, in fact, for them to heat through if you try to cook them while frozen. But, because it's such a small cut, you can defrost in hot water in a matter of minutes and then cook it in just a few minutes more.
Run your hot tap water for a minute or two so it heats fully. Fill a large bowl with hot water and submerge the frozen ahi tuna steak while it's still wrapped in airtight packaging. Leave it for about 10 minutes to thaw.
Place a cast iron or other heavy bottomed skillet on the stove over high heat for a few minutes.
Unwrap the tuna steak and pat it dry with paper towels while the pan heats. Brush both sides with a little cooking oil, then sprinkle on salt and pepper to taste. Pat on other flavoring agents, if you like, such as thyme, dill, tarragon, rosemary, lemon or lime zest or a touch of chili pepper powder or Cajun seasonings.
Place the ahi tuna in the pan and sear it for 2 to 3 minutes for rare to medium-rare. Turn it over with a spatula or tongs. Cook the other side for another 2 to 3 minutes for rare to medium-rare. Turn the tuna steak again and cook both sides once more for another 1 to 2 minutes each for medium or medium-well. Remove it from the pan promptly.