Prepare the Chicken: Season the chicken with salt and pepper, then toss with cornstarch until evenly coated.
Fry to Perfection: Heat vegetable oil in a large pan over medium-high heat. Fry the chicken cubes in batches until golden and crispy. Remove and set aside.
Sauté the Vegetables: In the same pan, sauté the onion, red bell pepper, and yellow bell pepper until slightly softened but still crisp.
Make the Sauce: In a bowl, whisk together ketchup, cider vinegar, brown sugar, soy sauce, ginger, garlic, and reserved pineapple juice. Pour into the pan with the vegetables.
Combine Everything: Add the crispy chicken and pineapple pieces to the pan, tossing to coat in the sauce. Simmer for 3-5 minutes until everything is heated through and the sauce thickens.
Serve and Enjoy: Serve immediately over rice or noodles for a deliciously sweet and tangy meal.