Cut tuna into ¼ to ½-inch thick slices. Then using a cookie cutter or knife, cut into rounds about 1-inch in diameter. (You may leave the slices as rectangles or squares too – that just gives you another shape of hamburger, or more like sandwich) Cut lettuce into small pieces or shreds. Prepare a serving plate. For each burger, you would need two pieces of tuna and about 1 tablespoon of rice. Place a tuna piece at the bottom, then some rice on top of it (You can squeeze the rice into a patty by hand. You can also use the same cookie cutter that you used to cut sashimi to make the rice into the same size. Push the rice in then remove the cookie cutter.) Put some lettuce on the rice, add 2 drops of soy sauce, and then cover with another piece of tuna. Garnish with a little wasabi on the top layer.