Make sushi rice. Combine 1 Tbsp. kewpie mayonnaise and 1 tsp. Sriracha hot chili sauce.
Cut 1 sheet nori in half crosswise. Cut 4 oz (113 g) sashimi-grade tuna ¼ inch cubes.
Cut 2 green onions into thin rounds. Take a bowl to combine tuna, sesame oil, 1 Tbsp. sriracha hot chili sauce and green onions. Mix well.
In a bowl, combine ¼ cup water and 1 Tbsp. rice vinegar applying the mixed water to your hands. Place a sheet nori shiny side down on a bamboo mat.
Spread ¾ cup of rice evenly onto the nori sheet.
Sprinkle white sesame seeds.
Turn the sheet of nori over so that the rice side is facing down. Place half of the tuna mixture at the bottom end of the nori sheet. Roll into a tight cylinder press gently.
Cut the roll in half then cut each half into 3 pieces. Put mayo on top. Garnish with green onion and serve.