Rinse the rice thoroughly until the water runs clear, then cook it with water. In a separate pan, heat vinegar, sugar, and salt until dissolved. Once the rice is cooked, gently fold in the vinegar mixture and sesame seeds.
Grease your baking dish lightly with sesame oil or spray. Spread the seasoned rice evenly and press it gently to form a flat layer. Sprinkle a generous amount of furikake seasoning over the top for added flavor.
Finely chop or shred the imitation crab and place it in a bowl. Mix it with mayonnaise and sriracha, adjusting spice levels to your taste. Add the egg and stir well to combine into a creamy mixture.
Spread the crab mixture evenly over the rice layer. Smooth it out gently to cover the entire surface. Sprinkle black sesame seeds and an extra layer of furikake on top.
Mix mayonnaise and sriracha to create a spicy sauce. Drizzle it over the casserole using a spoon or piping bag. Bake at 350°F (175°C) for about 25 minutes until heated through and slightly golden.