Rinse the short-grain rice under cold water until the water runs clear, about 2 minutes.

Combine the rinsed rice and 1 1/4 cups water in a rice cooker or pot and cook for 20 minutes, or until the water is fully absorbed.

While the rice cooks, mix the drained tuna, mayonnaise, soy sauce, and sesame oil in a mixing bowl until well combined.

Once the rice finishes cooking, let it steam in the closed cooker or pot for 10 minutes.

Transfer the rice to a bowl, season lightly with salt, and fluff gently with a spatula.

Wet your hands in a small bowl of water, then take about 1/2 cup of rice and flatten it slightly in your palm.

Place a spoonful of the tuna mixture in the center of the rice, then fold and shape into a firm triangle or ball, about 2 inches across.

Wrap each onigiri with a quarter sheet of nori, if using, and trim any excess with a sharp knife.