Rinse the sushi rice thoroughly until the water runs clear. Combine the rinsed rice and water in a medium pot, bring to a boil, then reduce heat, cover, and simmer for 15 minutes. Remove from heat and let stand, covered, for another 10 minutes.

While the rice cooks, whisk rice vinegar, sugar, and salt together until dissolved. Transfer the steamed rice to a large bowl, pour the vinegar mixture over it, and gently fold. Let the rice cool slightly.

In a separate bowl, combine diced salmon, mayonnaise, Sriracha, soy sauce, sesame oil, and sliced green onions. Mix gently until the salmon is evenly coated.

Preheat oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish. Press the seasoned sushi rice evenly into the bottom of the dish as the base. Spread the spicy salmon mixture over the rice layer, distributing it evenly. Sprinkle furikake generously over the salmon, followed by sesame seeds. Bake for 15-20 minutes until golden brown and salmon is cooked through and slightly caramelized. Remove from oven and cool for a few minutes before garnishing with extra green onions, spicy mayo, nori squares, diced avocado, and cucumber slices for serving.