Toss chicken with cornstarch, flour, salt, and pepper until evenly coated.
Heat oil over medium-high heat and fry chicken in batches until golden and crispy. Drain and set aside.
In a clean wok or skillet, heat vegetable oil. Sauté garlic, ginger, and chilies until fragrant.
Add onion and bell pepper; stir fry briefly until slightly tender but still crisp.
Stir in soy sauce, chili sauce, vinegar, sugar, and water. Simmer until glossy.
Add crispy chicken and toss quickly to coat evenly.
Remove from heat and serve immediately.