Prepare the Meatballs: In a large bowl, combine ground beef, breadcrumbs, egg, garlic, soy sauce, salt, and pepper. Mix well and shape into 1-inch meatballs (about 18–20 meatballs).
Cook the Meatballs: Heat a large skillet over medium heat. Add a tablespoon of oil and cook the meatballs until browned on all sides (about 8–10 minutes). Remove from the skillet and set aside.
Make the Sweet and Sour Sauce: In the same skillet, add bell peppers and cook for 2–3 minutes until slightly softened. Add pineapple chunks (with juice), ketchup, vinegar, brown sugar, and soy sauce. Stir well. Bring to a simmer and cook for 3–4 minutes. Add the cornstarch slurry and stir until the sauce thickens (about 1–2 minutes).
Combine and Serve: Return the meatballs to the skillet and toss to coat with the sauce. Simmer for another 2–3 minutes until the meatballs are heated through. Serve over rice or noodles.