Prepare the rice: Place the rice in a fine-meshed sieve and rinse well under cold water, rubbing the grains of rice between your fingers, until the water runs clear. This helps remove any excess starch and improves the texture of the rice once cooked. Drain well.

Combine the rice and water, and cook in a rice cooker according to the manufacturer’s directions. If you do not own a rice cooker, combine the rice and water in a medium (non-stick, if possible) saucepan. Bring to a boil, reduce heat to low, and cook covered for 16 to 18 minutes or until the water has been completely absorbed. Remove from heat, gently fluff the rice with a rice paddle or wooden spoon, place a clean kitchen towel over the pot, top with the lid again, and let stand covered for 10 minutes. While the rice cools slightly, prepare the onigiri filling.

Make the filling: In a medium bowl, combine the albacore tuna, mayonnaise, hot sauce, and rice vinegar with a fork until evenly combined. Stir in the scallions.

Assemble: Set aside a medium bowl of room temperature water and a small bowl of kosher salt for pinching. Once the sushi rice is cool enough to handle, start assembling the onigiri by hand.

Lightly moisten your hands by dipping them in the water bowl. Sprinkle both of your palms lightly with kosher salt and rub them together. Grab roughly a ½ cup of sushi rice and gently press into a thick oblong shape in one palm of your hand. Add a tablespoon of the tuna filling, pinched together with your fingertips, to the center of the rice and close the rice over the filling.

Gently shape the rice ball into a wide triangular shape using the palms of one hand and the fingers of your other hand, rotating the rice ball as you go. Avoid compressing the rice too hard, it should hold together well but be lightly packed. Sprinkle the outside of the onigiri with black sesame seeds. Wrap each onigiri in a nori sheet and place an additional small spoonful of tuna filling on the top of each onigiri. Serve immediately.