Combine rice, 2 1/2 cups water, sesame seeds, and 1 teaspoon salt in a large nonstick pot over medium heat; bring to a boil. Cover and cook until water is absorbed and rice is soft and sticky, about 15 minutes. Stir in soy sauce.
Mix 1 cup water and 1 tablespoon salt in a small bowl until salt is dissolved.
Place a 12-inch piece of plastic wrap in a small teacup, molding the plastic to fit the cup and letting the ends hang over the edges. Moisten plastic wrap with some of the salted water.
Place a 2x5-inch strip of nori seaweed halfway into the cup. Scoop some rice into the cup, leaving a 1/2-inch gap at the top. Make a small indent in the rice with a spoon; add a spoonful of tuna. Cover tuna with a spoonful of rice and fold over the exposed end of the seaweed strip.
Gather the ends of the plastic wrap tightly and remove the rice ball from the cup. Shape and compress the ball tightly with your hands. Transfer to a large plate. Repeat with remaining nori seaweed, rice, and tuna. Refrigerate onigiri until firm, 8 hours to overnight.
You can be very creative with these - they're the Japanese equivalent of a sandwich. If you don't like seafood or seaweed, you can just make them plain. Here are some other combinations you might try instead of seaweed and tuna:
Cook the rice with taco seasoning and use shredded chicken as a filling
Cook the rice with 2 eggs and use ketchup as a filling
Add a few tablespoons of curry paste and use steak or lamb as a filling
Fill with vegetable cream cheese, cucumber, and avocado.
Per Serving: 298 calories;2.1 g fat;
54.1 g carbohydrates;
13.8 g protein;
10 mg cholesterol;
1875 mg sodium.