Sprinkle enough salt and pepper all over the tuna steaks. Let it stand for 5 minutes.
Heat a non-stick flat pan or grill pan. Lightly spray cooking oil on the pan to prevent the steaks from sticking.
Sear each side of the tuna steaks for 1 to 2 minutes per side. If you want your tuna steak to be well done, add more searing time. Remove the steaks from the pan and place in a clean plate.
Meanwhile combine the mayonnaise (white mayo) with the sesame oil. Mix well. Spread 1 tablespoon of the mayonnaise mixture on both sides of the Kaiser or hamburger buns. Arrange the chopped lettuce and sliced tomato on one bun. Top with the seared ahi tuna steak.
Spread the wasabi mayonnaise over the ahi tuna steak. Top with the other slice of the bun.
Serve. Share and enjoy!