Coat the interior of the slow cooker insert with a non-stick spray, then add the frozen meatballs directly into the base. In a separate mixing bowl, whisk together the beef broth, brown gravy mix, onion soup mix, Worcestershire sauce, and ketchup until thoroughly blended. Pour this seasoned mixture over the meatballs and stir gently to ensure all are well-coated. Secure the lid on the slow cooker and set it to cook on low for 6 hours, stirring occasionally throughout the cooking process. Once cooking is complete, create a slurry by combining the cornstarch and cold water in a small bowl, stirring until smooth. Incorporate the slurry into the slow cooker, mixing until the sauce thickens and evenly coats the meatballs. Serve immediately over mashed potatoes or egg noodles. For a fresh finish, garnish with chopped parsley if desired.