Make two cuts on each side of the fish. Rub lightly with salt and pepper. Put fish on a metal or heatproof dish and scatter salted mustard green, mushroom and ginger slices on and around the fish.
Break up the sour plum and distribute it over the fish and arrange tomato slices around fish.
Steam fish over rapidly boiling water for 12 to 15 minutes or until the eyes of the fish pop out. (This is an indication that the fish is cooked.) Pour combined oil over the fish immediately, add garnishing and serve.