Peel and julienne the ginger. Trim and slice the scallions on a bias. Place in shallow, heatproof dish large enough to hold the fish.
Make sure the fish is clean of scales and snip off the sharp spines with kitchen shears.
Score the fish on both sides on a bias at 2-inch (5cm) intervals, down to the bone. Season lightly with salt and pepper inside and out. Place the fish in the dish and disperse the scallion whites (reserve the greens for garnish) and ginger on top, underneath, and inside the fish. Add the soy sauce, peanut oil, sesame oil and the sugar.
Place the dish with the fish set on a rack above simmering water in a wide, shallow pan with a tight fitting lid. Cook until a small knife or skewer can be easily inserted into the thickest part of the body of the fish, 8 to 10 minutes. Remove from the pan, sprinkle with scallion greens and serve immediately.