Make the Gravy Base
In a bowl, whisk together gravy mix, cream of mushroom soup, beef broth, onion soup mix, garlic powder, and black pepper until smooth.
Add Meatballs
Place frozen meatballs into the slow cooker.
Pour Sauce
Pour gravy mixture over the meatballs and gently stir to coat.
Cook
Cover and cook on LOW for 4–5 hours or HIGH for 2–3 hours, until heated through.
Thicken (Optional)
Mix cornstarch with cold water. Stir into slow cooker and cook an additional 20–30 minutes on HIGH.
Serve
Stir gently and serve hot over mashed potatoes, noodles, or rice.