Place the sliced mushrooms and onions in the bottom of a 6-quart crock pot sprayed with nonstick spray. Then pour frozen meatballs over the top, no need to thaw them.
Add beef broth, gravy mix, Worcestershire sauce, ketchup, mustard, black pepper, and garlic in a medium bowl. Whisk until well combined and pour over meatballs. Cover and cook on high for 2-3 hours or low for 5-6 hours. (If your gravy isn't thick enough for your liking, whisk cornstarch and water together in a small bowl and pour over the meatballs and mix until well combined. Cook for ten minutes or until the gravy thickens.) Serve over rice or mashed potatoes, and enjoy!
