In a large bowl, combine meat, bread crumbs, egg, Parmesan, onion, 1/2 teaspoon salt, and 1/2 teaspoon black pepper. Shape mixture into 1-inch meatballs.
Place meatballs in the slow cooker.
In a mixing bowl, combine marinara ingredients and pour over meatballs. Cover and cook on low 4-6 hours or until meatballs are cooked through.