01 - Rinse the Japanese short-grain rice under cold water until the water runs clear. Combine rice and water in a rice cooker or saucepan. Cook according to rice cooker instructions or bring to a boil, then cover and simmer for 15 to 18 minutes. Allow the rice to rest, covered, for an additional 10 minutes.
02 - In a bowl, thoroughly mix the drained tuna, mayonnaise, soy sauce, lemon juice, and wasabi paste if desired until the mixture is well combined.
03 - While the rice is still warm but manageable, moisten hands with water and sprinkle lightly with salt to prevent sticking.
04 - Take about 1/2 cup of cooked rice and gently flatten it in your palm. Place 1 tablespoon of tuna filling in the center.
05 - Carefully mold the rice around the tuna filling, shaping each into a tight triangle or ball, pressing firmly to seal the edges.
06 - Wrap a strip of nori around each shaped rice ball, positioning it at the bottom or along the side.
07 - Repeat the process with the remaining rice and filling. Serve immediately or wrap tightly and refrigerate for up to 24 hours.