Cut fillets into 2-inch and 3-inch pieces. Coat with cornstarch. Heat a large skillet (or wok, if you prefer) to 350 degrees F. Add oil, heat thoroughly. Fry fish on both sides until golden brown. While the fish is cooking, combine the sauce ingredients in a small bowl. Set aside. Remove the fish from the pan and quick-fry scallion, garlic, and ginger until golden brown. Add the sweet-sour sauce and bring to a boil. Stir constantly until thick. Reduce the heat to low. Add the fish gently into the sauce and simmer for 1 minute. Serve hot.