Bring water in a steamer to a boil, lower heat to a simmer. Layer ingredients in the steamer; cabbage, break the slice up a little, the slice of onion, broken into rings; sprinkle with salt and pepper to taste. Sprinkle 1 tablespoon cheese over evenly, add the slice of tomato. Season the fish with salt and pepper and place the fish fillets evenly over the tomato and top with the other tablespoon of cheese.
Steam for approximately 10 minutes or until the cabbage is tender crisp and the fish is flaky. Try to retain the layers to remove to plate.