Prepare the Chicken:
Coat the Chicken: Toss the chicken pieces in cornstarch, ensuring they are evenly coated. Dip the coated chicken into the beaten eggs.
Fry the Chicken: Heat the vegetable oil in a large skillet over medium-high heat. Fry the chicken pieces in batches until golden brown and crispy (about 4-5 minutes per side). Remove and drain on paper towels.
Prepare the Sauce:
Make the Sauce: In a small saucepan, whisk together the ketchup, rice vinegar, sugar, and soy sauce. Bring the mixture to a simmer over medium heat. Stir in the cornstarch slurry and cook for 1-2 minutes until thickened. Remove from heat and set aside.
Stir-Fry the Vegetables:
Cook the Vegetables: In the same skillet used for the chicken (with excess oil removed), stir-fry the bell peppers, pineapple chunks, and onion over medium heat for 3-4 minutes, until slightly softened.
Combine and Serve:
Combine Chicken and Sauce: Return the fried chicken to the skillet with the vegetables. Pour the sweet and sour sauce over the chicken and stir gently to coat everything evenly. Simmer for 2-3 minutes until heated through.
Serve: Serve hot over steamed rice or noodles.