Preheat the oven to 400 F and line a baking sheet with parchment paper.
Combine ground beef, onion, garlic, bread crumbs, egg, salt and pepper together in a large bowl using hands or a rubber spatula.
Form the meat mixture into approximately 24 1-inch meatballs and place them on the prepared baking sheet.
Bake in the preheated oven for 15-20 minutes until browned and cooked through.
Sweet and Sour Sauce
While the meatballs are baking, heat the olive oil in a large skillet over medium heat. Add the onion and garlic and cook until softened, about 2-3 minutes.
Whisk in the ketchup, brown sugar, Worcestershire sauce and vinegar.
Reduce the heat to low and simmer for about 5 minutes. Whisk in another tablespoon or two of water if the sauce is too thick.
Add the cooked meatballs and stir to coat. Serve over rice or mashed potatoes, or on their own as an appetizer.