Fill a wok or large pan with enough water to reach the bottom of a bamboo steamer, and bring the water to a boil over high heat. Place the bamboo steamer in the pan and cover to preheat.
Pat the fish dry inside and out with a paper towel and place on a plate. Stuff the cavity and top the fish with all of the mushrooms and half of the ginger and scallions.
Carefully lower the plate with the fish into the preheated steamer. Cover and steam the fish until the fish is cooked through and the flesh is opaque, 10 to 15 minutes.
Meanwhile, in a small bowl, combine the soy sauce, 2 tablespoons water and the sugar. Set aside.
Remove the fish and plate from the steamer and drain any excess liquid. Scatter the fish with the remaining ginger and scallions. Pour the soy sauce mixture over the fish, immediately followed by the hot oil. Serve with steamed white rice.
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