In a bowl, combine the vinegar, ketchup, sugar, ⅔ cup water, soy sauce, and 1 tablespoon cornstarch. Mix together and set aside.
Rinse the fish fillets and lightly pat them dry — make sure they remain slightly damp so the starch adheres well.Cut the fillets into large bite-sized pieces and gently score them. This creates more surface area for the coating, resulting in an even crunchier texture. Combine the cornstarch and baking powder in a shallow bowl. Lightly season the fillets with salt and pepper, then dredge each piece in the flour mixture, shaking off any excess.
In a wok or a skillet, add about an inch of oil and set it over medium-high heat. When the oil is hot, add the fish pieces carefully.
Fry them until crisp and golden brown for about 5 minutes, flipping halfway through. Set aside on a wire rack or paper towel to drain excess oil.
In the same wok or skillet, remove excess oil, leaving just enough to lightly coat the bottom. Turn the heat to high and quickly stir-fry the bell peppers, onion, and pineapple, about 2 minutes or until slightly charred.
Push the vegetables to the side of the wok, then lower the heat to medium. Add more oil as needeed, then sauté the ginger, garlic, and red pepper flakes in the center for 30 seconds.
Add the scallions, then toss everything together. Lightly season the vegetables with salt and pepper.
Turn the heat up to high and add the sweet and sour sauce. Let it slighty thicken for about a minute or two.
Carefully place the fish into the sauce, then turn off the heat. Gently toss to evenly coat them with the sauce. Season with more ground pepper, as desired.
Enjoy your Sweet and Sour Fish right away to ensure the fish remains deliciously crisp. Serve with a side of steamed rice for a complete and satisfying meal.