Marinate the fish for about 10 minutes. Mix the sauce ingredients in a small bowl and set aside.
Heat up the wok with cooking oil. Add ginger slices into the wok when the oil is very hot. Stir-fry the ginger until aromatic and add in the fish fillet. Stir-fry the fish until they are half cooked. Add in the sauce and continue to stir-fry until the fish is cooked through. If the sauce dries out, add in a little water. Add in the chopped scallions and do a few quick stirs, dish out and serve hot.