Rinse the sushi rice under cold water until the water runs clear. Drain.
Cook the sushi rice according to your rice cooker's instructions, or in a pot with 2 ½ cups of water, bringing to a boil and then simmering covered for 18-20 minutes. Let it sit covered for an additional 10 minutes.
In a small bowl, mix the rice vinegar, sugar, and salt until dissolved. Gently fold this mixture into the cooked rice, allowing it to cool slightly.
While the rice is cooling, prepare the tuna. In a mixing bowl, combine the diced tuna, soy sauce, sesame oil, and sriracha (if using). Mix well and set aside.
Once the rice has cooled, divide it evenly among the serving bowls.
Top each bowl of rice with the marinated tuna, then arrange the avocado, cucumber, edamame, seaweed salad, and green onions on top.
Sprinkle sesame seeds over each bowl for garnish. Add pickled ginger on the side if desired.