Score the fish on both sides from head to tailas if to fillet and debone it, but do not cut through.
Rub salt on both sides of the fish and set in a heat proof dish for steaming.
Insert 4-5 slices of ginger into fish belly.
Arrange the tomatoes, sour plum, tofu and mushrooms around the fishand place julienned ginger slices on top.
In a small bowl, mix all the ingredientsfor the sauce and set aside
Scatter the preserved mustard on top and around the fish.
Pour sauce over the fish and then steam the dish for 15 min or until fish is thoroughly cooked.
Remove from steamer.
Garnish and serve with steamed rice.