01 - Rinse the Japanese short-grain rice under cold running water, gently swirling with your hands. Repeat several times until the water runs clear, then drain thoroughly.
02 - Combine the rinsed rice and 2 1/2 cups water in a rice cooker or heavy-bottomed pot. Cook according to package instructions. Once done, let the rice rest covered for 10 minutes to allow the grains to firm up.
03 - While the rice rests, drain the canned tuna well and place it in a mixing bowl. Add the Japanese mayonnaise, soy sauce, and black pepper. Fold together until the mixture is creamy and evenly combined.
04 - Once the rice is still warm but cool enough to handle comfortably, wet your hands with water and rub a thin layer of salt across your palms. This prevents sticking and adds a light seasoning to the exterior.
05 - Scoop approximately 1/2 cup of warm rice into your palm and flatten it into a round disc. Place a generous spoonful of the tuna mayo filling in the center, then gently fold the rice over the filling. Cup your hands to shape it into a triangle or oval, pressing firmly enough to hold together without compacting the rice.
06 - Repeat the shaping process with the remaining rice and filling. Wrap a strip of nori around the base or center of each onigiri. Serve immediately or pack for later.