Season the chicken pieces lightly with salt and black pepper.
Coat the chicken in cornstarch, then dip into beaten eggs, ensuring all pieces are well coated.
Heat vegetable oil in a deep skillet or wok over medium-high heat.
Fry the chicken in batches until golden and cooked through. Remove and drain on paper towels.
In a bowl, whisk together pineapple juice, ketchup, vinegar, sugar, and soy sauce.
Pour the sauce mixture into a clean skillet or wok and bring to a gentle simmer.
Stir in the cornstarch slurry and cook until the sauce thickens and becomes glossy.
Add pineapple chunks, bell peppers, and onion to the sauce and cook for 2–3 minutes until slightly tender.
Add the fried chicken to the pan and toss until evenly coated with sauce.
Remove from heat and serve immediately.