Place 1 (26-ounce) bag frozen meatballs and 8 ounces sliced mushrooms in the bottom of the crock pot.
In a medium bowl, whisk together 2 cups low-sodium beef broth, 2 tablespoons Worcestershire sauce, 1 (1-ounce) package onion soup mix, and 2 tablespoons ketchup.
Pour the broth mixture over the meatballs and cook on HIGH for 2 hours or LOW for 4 hours.
In a small bowl, add 2 tablespoons cornstarch and 2 tablespoons water, creating a slurry. Stir into the broth and let cook for a few minutes, until thickened.
Garnish with fresh chopped parsley and serve over mashed potatoes.
Notes
Leftover Instructions
Refrigerator: Store cooled meatballs and gravy in an airtight container in the fridge for up to 4 days.
Freezer: Freeze in a freezer-safe container for up to 3 months. Thaw overnight in the refrigerator before reheating.
Reheat: Warm on the stove over medium-low heat or in the microwave until heated through. Add a splash of beef broth if the gravy is too thick.