Prepare the Slow Cooker: In the bottom of the slow cooker, add the frozen meatballs, evenly distributing them across the base.
Make the Sauce: In a mixing bowl, combine the beef broth, onion soup mix, Worcestershire sauce, and mushrooms. Stir to combine.
Pour Over Meatballs: Pour the prepared sauce mixture over the meatballs, ensuring they’re well-covered.
Cook: Cover and cook on low for 4–5 hours or on high for 2–3 hours until the meatballs are heated through and the flavors are well blended.
Thicken the Gravy: In a small bowl, whisk together the cornstarch and water to form a slurry. Stir the slurry into the slow cooker during the last 30 minutes of cooking, allowing the gravy to thicken.
Season and Serve: Once the gravy has thickened to your liking, season with salt and pepper as needed. Garnish with fresh parsley, if desired.