Steps
1. Cut the pork tenderloin into bite-sized pieces and season with salt and pepper.
2. In a bowl, mix soy sauce, vinegar, sugar, ketchup, and cornstarch to create the sauce.
3. Heat vegetable oil in a pan over medium heat and stir-fry the pork until browned and cooked through.
4. Add the diced bell pepper, onion, and pineapple chunks to the pan, stir-frying for an additional 2-3 minutes.
5. Pour the sauce over the pork and vegetables, stirring well to combine. Cook until the sauce thickens slightly.
6. Serve hot over rice or noodles.