Sprinkle salt and white pepper on both sides of the fish fillet and set in a deep dish.
Arrange the tomato wedges, sour plum and tofu around the fish, and place the ginger slices on top.
In a small bowl, mix all the ingredients for the sauce and set aside.
In a sauce pan, heat up the cooking oil and add in the chopped garlic. Stir-fry until aromatic but not burnt.
Then add the pork, and stir-fry until half-cooked.
Lastly, add in the preserved mustard slices and continue to stir-fry for 1 minute.
Pour the sauce into the pan, let it come to a boil and turn off the heat.
Scatter the pork and preserved mustard on top and around the fish.
Pour the gravy over the fish and then steam the dish for 6 minutes or until the meat is thoroughly cooked.
Garnish and serve with steamed rice.