In a large bowl, combine the flour, cornstarch, garlic powder, ginger powder, salt, and black pepper. Gradually add water to form a smooth batter.
Add the chicken pieces to the batter, ensuring they are well coated.
In a large skillet or frying pan, heat the vegetable oil over medium-high heat. Once hot, add the battered chicken pieces in batches, frying until golden brown and cooked through, about 5-7 minutes. Remove and drain on paper towels.
In another pan, heat 2 tablespoons of vegetable oil over medium heat. Add the chopped onion and bell pepper, sautéing until softened, about 3-4 minutes.
Stir in the minced garlic and ginger, cooking for an additional minute.
In a small bowl, mix together the soy sauce, ketchup, sugar, vinegar, and red chili flakes. Pour this sauce over the sautéed vegetables and bring to a simmer.
Add the fried chicken pieces to the sauce, tossing to coat evenly. Cook for an additional 2-3 minutes to heat through.
Serve hot, garnished with chopped green onions.