Prepare the slow cooker: Place frozen meatballs into the slow cooker insert.
Make the gravy base: In a medium bowl, whisk together beef broth, brown gravy mix, onion soup mix, ketchup, and Worcestershire sauce until smooth.
Pour and cook: Pour the gravy mixture over the meatballs, making sure they are evenly coated. Cover and cook on low for 4–5 hours or high for 2–3 hours until meatballs are heated through.
Thicken the sauce: About 20 minutes before serving, mix cornstarch and cold water to make a slurry. Stir into the slow cooker, cover, and cook until the gravy has thickened.
Serve: Spoon meatballs and gravy over mashed potatoes, rice, or noodles. Garnish with parsley if desired.