Prepare the Chicken:
In a bowl, mix flour, cornstarch, salt, and pepper. Dip chicken pieces into beaten eggs, then coat with the flour mixture.
Fry the Chicken:
Heat vegetable oil in a large skillet or wok over medium-high heat. Fry chicken in batches until golden brown and cooked through, about 4–5 minutes per batch. Drain on paper towels.
Make the Sweet and Sour Sauce:
In a small saucepan, combine pineapple juice, rice vinegar, ketchup, brown sugar, and soy sauce. Bring to a simmer over medium heat. Stir in cornstarch slurry and cook until thickened, about 1–2 minutes.
Stir-Fry Vegetables:
In a large skillet or wok, heat 1 tbsp vegetable oil over medium-high heat. Add bell peppers and pineapple chunks and stir-fry for 2–3 minutes until slightly tender-crisp.
Combine Chicken and Sauce:
Add the fried chicken to the skillet with vegetables. Pour the sweet and sour sauce over everything and toss to coat evenly. Cook for 1–2 minutes until heated through.
Serve:
Serve hot over steamed rice or noodles. Garnish with sesame seeds or sliced green onions if desired.