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 over medium-high heat 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 Rice Balls
Combine the tuna, mayonnaise and sriracha in a small bowl. Mix well. Set aside.
Crush the nori: place nori sheets into a food processor and pulse until fine. Alternatively, place nori into a Ziploc bag and crush them by hand.
Combine the cooked rice, sesame seeds, soy sauce and sesame oil in a large bowl. To make the vegetarian rice balls: with wet hands, form golf ball-sized portions with the rice mixture. Roll in nori flakes.
To make the tuna-filled rice balls: with wet hands, form rice into a small disk. Top with a small amount of tuna mixture. Place another disk of rice over the tuna and roll the rice ball to seal the edges.