Start by rinsing the rice under cold water until the water runs clear to remove excess starch. Cook the sushi rice in water until it becomes soft, fluffy, and slightly sticky. Warm the rice vinegar, sugar, and salt until dissolved, then fold it gently into the hot rice, finishing with the toasted sesame seeds.
Grease your baking dish lightly with sesame oil or spray. Press the seasoned sushi rice evenly into the bottom of the dish to form a firm base. Sprinkle a generous layer of furikake seasoning on top to give the rice a savory, ocean-inspired flavor.
Finely chop or process the imitation crab to achieve a flaky texture. Mix it with Kewpie mayo, sriracha, and a beaten egg, blending until creamy and smooth. Spread the crab mixture evenly over the rice layer, then sprinkle black sesame seeds and a bit more furikake on top.
Whisk together the remaining mayo and sriracha in a small bowl. Drizzle it over the crab mixture in zig-zag lines for even coverage. Bake the sushi casserole at 350°F (175°C) for about 25 minutes, or until hot, bubbly, and lightly golden.