Make Sushi Rice: Wash the rice with cold water until the water runs clear. Then add the rice and water to the rice maker. Cook according to the instruction.
Once cooked, transfer the hot rice to a large bowl and let it cool down slightly. When it’s still very warm, stir in the sushi vinegar (or the mixture of rice vinegar, sugar, and salt).
Prepare the Tuna: Cut against the grain, about 30 to 45 degree angle. Cut your tuna into thin slices. I recommend cutting it to 3-inch long, 1-inch wide, and 1/4-inch thick.
Shape the Rice: Take about 3 tablespoons of rice in your right hand. Squeeze it together until it rolls into a firm “log” sized oval shape. Try to make the bottom flat with the rounded top. (Don’t squeeze too tight, and it should still contain a little bit of air in the rice.)
Assemble the Nigiri: Place the tuna slice on the root of your fingers. Then take a pea-sized portion of wasabi, and spread in the middle of the fish.
Place the rolled sushi rice onto the tuna, and bend your fingers to cover both the fish and rice together. Press down the rice using the index finger from the other hand.
Turn the fish and rice and let the tuna slice sit on top of the rice. Serve tuna nigiri with optional wasabi, soy sauce, along with pickled ginger.