Rinse the rice under cold water until the water runs clear. Cook the rice according to the package instructions.
While the rice is still hot, gently fold in a mixture of rice vinegar, sugar, and salt. Allow it to cool to room temperature.
Preheat your oven to 375°F (190°C).
In a large bowl, combine the shredded imitation crab, mayonnaise, soy sauce, sesame oil, and Sriracha (if using). Mix well until everything is evenly coated.
In a baking dish, spread half of the rice as the base layer.
Add the crab mixture (from step 4) on top, spreading it evenly. Add the remaining rice as the top layer. Arrange sliced avocados on top and sprinkle with green onions and sesame seeds.
Place the baking dish in the preheated oven and bake for about 20-25 minutes or until the top is golden and slightly crispy.
Cut the sushi bake into squares, similar to how you would cut a lasagna. Serve with nori sheets on the side for that authentic sushi experience.