Place fish on a flat dish that will fit on a rack or in a large bamboo steamer. Mix soy sauce, peanut oil and cornflour in a small bowl. Season with pepper and pour over fish.
Scatter mushrooms, ginger and snow peas over fish. Cover fish with foil or steamer lid and steam over a wok or pan of boiling water for 5 minutes, or until cooked. Remove fish from steamer, and serve immediately with steamed rice.