Prepare the Chicken: In a bowl, season the diced chicken with salt and pepper. Toss with cornstarch until evenly coated.
Fry the Chicken: Heat vegetable oil in a skillet or wok over medium-high heat. Fry the chicken in batches until golden and crispy, about 5-6 minutes. Remove and set aside.
Sauté the Vegetables: Remove excess oil from the pan, leaving about 1 tablespoon. Add the onion, red bell pepper, and yellow bell pepper. Stir-fry for 3-4 minutes until slightly softened.
Make the Sauce: In a bowl, whisk together ketchup, cider vinegar, brown sugar, soy sauce, ginger, garlic, and the reserved pineapple juice. Pour into the pan with the vegetables and bring to a simmer.
Combine Everything: Add the cooked chicken and pineapple pieces to the pan, tossing to coat in the sauce. Simmer for 2-3 minutes until the sauce thickens.
Serve: Enjoy hot over steamed rice or noodles.