Soak shiitake mushrooms in hot water until soft, 5 to 10 minutes.

Meanwhile, set an oven rack about 6 inches from the heat source and preheat the oven’s broiler. Lightly grease a 9x13-inch baking dish with cooking spray.

Spread rice in the prepared baking dish; sprinkle furikake evenly over the top.

Drain mushrooms and squeeze out excess water. Mix mushrooms, imitation crabmeat, mayonnaise, sour cream, kamaboko, and tobiko together in a large bowl until well combined. Spread over furikake.

Broil in the preheated oven until lightly browned on top, about 15 minutes. Remove from the oven and slice into 24 portions while in the baking dish.

Spoon a generous portion onto a sheet of seasoned seaweed, wrap loosely, and enjoy immediately.