Fill a wok three-quarters of the way up with water and place over high heat. Bring the water to a simmer. Place the fish on a plate. Cut 5 or 6 slits into the skin on both sides. Sprinkle the fish with salt and ground white pepper. Place a few pieces of ginger into the slits of the fish, and then stuff the cavity with the remaining ginger and the shiitake mushrooms. Pour the rice wine over the fish, place the fish in a bamboo steamer and close the lid. Place the steamer over the simmering water in the wok and steam until the fish is cooked and the flesh flakes apart with a fork, 10 to 12 minutes. After 10 minutes, sprinkle the scallions and sliced wild mushrooms on top of the fish and steam for 2 minutes more before serving. Serve immediately with the rice.