Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine ground beef, breadcrumbs, onion, milk, egg, salt, and pepper.
Mix the ingredients until just combined; avoid overmixing.
Shape the mixture into meatballs, approximately 1 inch in diameter.
Place the meatballs on a baking tray lined with parchment paper.
Bake the meatballs in the preheated oven for 20 minutes, or until cooked through.
While the meatballs are baking, prepare the sweet and sour sauce in a saucepan.
Add pineapple chunks and diced bell peppers to the sauce.
Simmer the sauce for about 5 minutes to combine the flavors.
Once the meatballs are done, remove them from the oven and add them to the sauce.
Stir gently to coat the meatballs with the sauce.
Serve hot over rice or with toothpicks for an appetizer.