Place tuna steak in small resealable zippered bag and add marinade. Turn to coat. Let stand for 10-20 minutes, turning several times.
Place black and white sesame seeds on a plate. Remove tuna steak from marinade and dip into sesame seed mixture, patting into steak until all sides are coated.
Preheat grill on high. Place tuna steaks directly on grill and quickly sear, about 1-½ to 2 minutes per side. Medium rare has an internal temperature of 125 degrees.
Notes
What type of tuna should I use?
You always want to start with high-quality ahi tuna, sushi grade if possible. It also goes under the names yellowfin and bigeye tunas. Purchase it from a reputable fish store where you know it is fresh or buy it frozen. Frozen tuna is usually cut and frozen directly on the boat making it as good as fresh. Also, the flash freezing kills any bacteria that might be on the fish.
When choosing your fresh tuna steaks, make sure that the color is a rich pink or deep red, which depends on the depth at which the tuna is found, and has no dark patches.
Do I have to marinate my ahi tuna?
Marinating the tuna steak will help prevent the fish from drying out as it is seared. The marinade should be on the basic side, containing ingredients such as soy, honey and oil, so as not to cook the fish while sitting.
How long should I grill my tuna?
Seared tuna needs a very hot grill, preheated to high, before searing since the cooking time is so short. Usually only 2 minutes a side will provide you the pinkish red interior that is most recommended. If testing with a meat thermometer, is should read 125 degrees for medium rare.