Place the sliced mushrooms and onions in the bottom of a 6-quart crock pot sprayed with nonstick spray.

Pour frozen meatballs over the top, no need to thaw them.

In a medium bowl, add beef broth, gravy mix, Worcestershire sauce, ketchup, mustard, black pepper, and garlic. Whisk until well combined.

Pour the mixture over the meatballs. Cover and cook on high for 2-3 hours or low for 5-6 hours.

If your gravy isn't thick enough, whisk cornstarch and water together in a small bowl and pour over the meatballs. Mix until well combined and cook for an additional 10 minutes or until the gravy thickens.

Serve over rice or mashed potatoes.