1. Arrange an inverted pie plate in a deep 12-inch skillet and add 3 cups of water to the skillet. Cover the skillet and bring water to a boil.
2. Pat fillets dry and arrange on a heatproof plate that will fit into the skillet with a 1 inch clearance around the plate. Sprinkle ginger and 1/4 cup scallions evenly on top of the fish. Stir together soy sauce and oil in a small bowl and drizzle evenly over the fillets. Carefully transfer plate with fish to the plate in the skillet and cover tightly, then steam over moderately high heat until the fish is just cooked through, about 15-20 minutes. Carefully remove the plate from the skillet and sprinkle with the remaining 1/4 cup of scallions.
