Rinse the sushi rice under cold water until the water runs clear, then drain it well.
Combine the rinsed rice and water in a rice cooker or pot, then cook according to your rice cooker instructions or bring to a boil and simmer covered for 15-20 minutes on low heat until water is absorbed.
While the rice is cooking, preheat the oven to 375°F (190°C).
In a mixing bowl, combine the mayonnaise, soy sauce, sriracha, rice vinegar, sesame oil, chopped green onions, and half of the sesame seeds.
Once the rice is cooked, transfer it to a large baking dish and evenly spread it across the bottom, pressing down gently to form a compact layer.
Flake the cooked salmon into bite-sized pieces and spread it evenly over the rice layer.
Spoon the mayonnaise mixture over the flaked salmon, spreading it to cover evenly.
Sprinkle the remaining sesame seeds on top for extra texture, and add sliced avocado if desired.
Bake in the preheated oven for 20-25 minutes, or until the top is golden and bubbly.
Remove from the oven and let cool for a few minutes before slicing and serving.