Place 2 cups of uncooked sushi rice into the rice cooker. Pour in 3 cups of water. Turn on the rice cooker. Once the rice cooker turns itself off, let the rice steam for 10-20 minutes. Remove the lid and let the rice cool before forming the rice balls.
Stovetop Instructions
Bring 3 cups of water to a boil in a medium-sized saucepan. Stir in the 2 cups of sushi rice using a rice paddle or wooden spoon. Cover and reduce heat to low. Cook rice, covered, 15-20 minutes until the water is absorbed. Allow the rice to cool until you can safely handle it.
Making the Onigiri
Drain the tuna and flake it with a fork. In a small mixing bowl, combine the tuna, sriracha and mayonnaise. Set aside.
Seed and dice the cucumber. Set aside.
Set up a small bowl of water to dip your hands in. Lay a nori sheet down on a flat surface. Using wet hands, scoop up a small handful of rice and place it onto the center of the nori. Form it into a flat disc.
Top the rice with a spoonful of tuna and some cucumbers. Wet your hands again and get another small handful of rice. Place rice on top of the tuna. Form the rice into the shape of a hockey puck around the tuna.
Wet your hands again. Using your fingertips, moisten the corners of the nori. Fold the corners over the rice, starting with the top right corner, bottom left corner, top left corner, bottom right corner.