Set the oven to 375°F (190°C) so it can preheat while you prepare the dish.
Rinse the sushi rice under cold water until the water runs mostly clear. Cook it with the water in a rice cooker or pot until soft and fully done.
In a small bowl, stir together the rice vinegar, sugar, and salt until the sugar dissolves. Transfer the cooked rice to a bowl, fluff it gently, and fold in the seasoned vinegar mixture.
Press the prepared rice into an even layer in the bottom of a 9x13-inch baking dish. Let it sit for a few minutes to cool slightly.
In a separate bowl, combine the diced salmon, mayonnaise, Sriracha, sesame oil, and chopped green onions. Stir until the mixture looks creamy and evenly blended. Add extra Sriracha if you want more heat.
Spoon the salmon mixture over the rice and spread it across the top in an even layer.
Bake for 25 to 30 minutes, or until the salmon cooks through and the top turns lightly golden.
Take the dish out of the oven and let it rest for a few minutes. Finish with extra green onions, nori strips, and tobiko if using.
Serve warm and scoop into portions with a spoon.