Rinse fish in cold water; pat dry. Cut fish crosswise into 3/4- inch-thick slices. Arrange slices in a 10-inch pie plate. Pour orange juice over fish.
Drain juice from mandarin oranges into a small bowl, reserving fruit. Add soy sauce, sherry, oil, and ginger to mandarin orange juice; stir well. Pour juice mixture over fish. Arrange green onion tops over fish.
Place pie plate on rack in a steamer filled with water to within 1 inch of the top of rack. Bring to a boil; cover steamer. Reduce heat; simmer 12 minutes or until fish flakes easily when tested with a fork.
Remove pie plate from steamer; arrange mandarin orange sections on top of fish. Serve immediately over hot cooked rice.