Rinse the rice well and place it in your rice cooker. Add the salt, rice vinegar, cooking oil, and chicken stock powder. Add water to the 2-cup line and mix gently. Cook as usual.
Drain the tuna as much as possible. Mix it with mentsuyu, sugar, mayonnaise, and sesame oil. Set aside.
When the rice is done, fluff it with a rice paddle using a slicing motion. Shape the onigiri while the rice is still hot so it holds together.
Place plastic wrap on your counter. Add 1/3 cup of cooked rice and flatten it gently. Put the tuna filling in the center. Top with another 1/3 cup of rice. Wrap it up and shape it into a triangle. Press about 15 times, then flip and press 15 more times.
Cut aluminum foil about twice the width of the seaweed. If the foil is much longer than the seaweed, trim the length too. Keep it about the same size, up to half an inch larger. Place the half seaweed sheet in the center of the foil.
Fold the left and right sides about half inch away from the seaweed edges. This space becomes the pull tab, so it’s very important. Flip it over and tape across the center. On the top side, let the tape extend about half an inch for easy peeling. Set it aside.
Place the foil sheet down with the tape side facing away from you. Put the onigiri on the seaweed side, centered. Seal the left and right sides snugly. Fold the top corners down and tape them to hold everything in place.
To eat, peel the tape from the top and pull it straight down. Gently open the sides and unwrap.
