Set a large bamboo or metal steamer basket over a skillet of simmering water over medium heat.
Crush the ginger slices with the flat side of a knife. Place the garlic and half each of the ginger and scallions on a plate that will fit inside the steamer. Score the fish skin a few times with a knife; season with salt and pepper. Place the fish skin-side up on the plate, drizzle with 2 teaspoons sesame oil and sprinkle with the sugar. Put the plate in the steamer. Mix the soy sauce and rice wine and pour over the fish.
Cover and steam the fish until just cooked through, 6 to 12 minutes, depending on the thickness. Carefully remove the hot plate. Add the snow peas to the steamer, season with salt, cover and cook until bright green, 1 to 2 minutes.
Transfer the fish to a platter, spoon the juices on top and sprinkle with the remaining scallions. Heat the remaining 2 teaspoons sesame oil and the peanut oil in a skillet over high heat. Add the remaining ginger and cook until it begins to brown. Pour the hot oil over the fish.