To prepare the sushi rice, rinse the rice in a fine-mesh strainer until the water runs clear.
Cook according to the package directions.
Meanwhile, heat the rice vinegar, sugar, and salt in a small saucepan over medium heat. Stir until the sugar is dissolved, then remove from the heat.
Transfer the cooked rice to a large bowl and mix gently with the vinegar mixture until fully coated, then cover the rice with a towel and let it cool while preparing the filling.
To prepare the tuna, first cut it into 1-inch cubes.
Add the tuna to a food processor and pulse 3-5 times, or until just chopped but not gummy or smooth.
Transfer the tuna to a bowl and combine it with Kewpie mayonnaise, sriracha, soy sauce, sesame oil, green onions, and sesame seeds.
Stir to combine.
To assemble the sushi, cover the bamboo mat with plastic wrap.
Cut the nori sheet to about 4 inches wide (or about 4 segments).
Add the nori sheet shiny side down.
Cover it with a thin layer of sushi rice.
Carefully flip the nori so that the sushi rice is facing down, towards the plastic wrap.
Spoon ¼ of the filling into a straight horizontal line in the center of the nori.
Use the mat to roll the rice and nori around the filling, creating a cylinder.