Heat the oven to 425°F and oil a rimmed sheet. Soak breadcrumbs in milk for 3 minutes, then mix gently with beef, pork, egg, salt, pepper and onion powder. Form 24 equal small meatballs.
Bake for 15 to 18 minutes until browned and the centers reach 160°F. Meanwhile, warm jelly, tomato paste, vinegar, broth and Worcestershire in a saucepan, whisking until smooth and steaming.
Pour hot glaze into a 4-quart slow cooker and fold in the hot meatballs. Cover and cook on HIGH for 2 hours, stirring gently after the first hour, until the centers reach 165°F and the glaze coats them.
Uncover for the final 10 to 15 minutes if the glaze is thinner than desired, stirring once to keep the edges from catching.
Switch to WARM for serving only if the food stays at least 140°F. Keep covered between servings and serve about 3 meatballs per appetizer portion.