Arrange 1/2 of the green onions on the bottom of the steaming bowl (it is important to steam in a container in order to retain the steam and juices around the fish). Place 1/2 of the mushrooms and Napa cabbage sections on top of the onions. Place fish on top of the vegetables. Sprinkle ginger, garlic, and red pepper flakes over fish. Top with the remaining green onions, mushrooms, and napa cabbage. Drizzle soy sauce and water over everything.
Place steam bowl in a steamer over 1 inch of boiling water, and cover. Steam for 15 to 20 minutes, or until fish flakes easily. Garnish with cilantro, if desired.
