Soak sour mustard for 30 minutes. Rinse, drain and cut into thin strips.
Combine all sauce paste ingredients in a small bowl. Mix well.
Place fish on a large plate. Sprinkle salt and ground pepper all over the fish.
Heat 4 tablespoons vegetable oil in a wok or large fry pan. Add prepared fish and fry until cooked and lightly brown, about 8 minutes (4 minutes on each side). Transfer fish to a serving dish.
Heat remaining 2 tablespoons vegetable oil in the wok or large fry pan. Add sauce paste and stir fry for 2 minutes.
Add prepared pickled mustard green. Stir fry for 30 seconds.
Pour in water. Stir to mix. Cover and bring to a boil. Allow it to simmer for 2 minutes.
Add cornstarch slurry. Stir to mix. Sauce will start to thicken. Turn off stove.
Ladle sauce over the fried fish. Garnish with green/spring onions and serve immediately.