Make the Meatballs: In a large mixing bowl, combine the ground beef or pork, breadcrumbs, egg, Parmesan cheese, chopped onion, minced garlic, salt, pepper, and parsley (if using). Thoroughly combine the ingredients, ensuring they are uniformly blended throughout the mixture.
Form and Fry the Meatballs: Shape the mixture into bite-sized meatballs, approximately one inch in diameter, for uniformity and easy cooking. Heat the olive oil in a large skillet over medium heat. Add the meatballs in batches and cook for 5-7 minutes, turning occasionally, until browned on all sides.
Prepare the Sweet and Sour Sauce: In the same skillet, add the ketchup, vinegar, brown sugar, soy sauce, Worcestershire sauce, and pineapple juice.
Add the Vegetables and Pineapple: Add the red and green bell peppers and pineapple chunks to the sauce. Cook for 2-3 minutes until the peppers start to soften slightly.
Thicken the Sauce: Stir in the cornstarch-water mixture and cook for another minute, or until the sauce thickens to your liking.
Combine Meatballs and Sauce: Return the golden-browned meatballs to the skillet, immersing them in the flavorful sauce. Simmer for 5-7 minutes, allowing the meatballs to absorb the sweet and sour flavors and heat through.
Serve: Serve the meatballs hot over rice or noodles, garnished with extra chopped parsley if desired.