Rinse the sushi rice under cold water until the water runs clear to remove excess starch. Drain well.
In a rice cooker or pot, combine the rinsed rice with 2 ½ cups of water. Cook according to the rice cooker instructions or bring to a boil, then reduce heat to low, cover, and simmer for about 20 minutes until tender. Remove from heat and let it sit covered for another 10 minutes.
While the rice is cooking, prepare the tuna. In a mixing bowl, combine the diced tuna, soy sauce, sesame oil, and rice vinegar. Gently mix and allow it to marinate for about 10 minutes.
Once the rice is cooked and fluffy, divide it evenly among four serving bowls as the base.
On top of the rice, arrange the marinated tuna, avocado slices, cucumber slices, edamame, and green onions. Sprinkle with sesame seeds.
If desired, serve with seaweed salad, pickled ginger, and a small dollop of wasabi on the side.