Preheat the oven to 350 F and line a baking sheet with foil.
In a medium-sized bowl, mix the ground beef, onion, breadcrumbs, egg, milk, Parmesan cheese, parsley flakes, ground mustard, salt, and pepper. With your hands wet, form the mixture into meatballs.
Place the meatballs on the prepared baking sheet and bake for 30 minutes or until evenly browned.