Start by spraying the crockpot with non-stick cooking spray.
Add the frozen meatballs to the slow cooker. Top with sliced onions and mushrooms.
In a bowl, whisk together the cream of mushroom soup, gravy mix, onion soup mix, beef broth, Worcestershire sauce, and Dijon mustard. Pour the mixture evenly over the meatballs.
Cover and cook on low for 7-8 hours or high for 3-4 hours, until everything is hot and bubbly. Stir well at the halfway point to mix the ingredients.
If you prefer a thicker gravy, mix cornstarch and water together, then stir into the slow cooker during the last 15–20 minutes of cooking.
Spoon the meatballs and gravy over mashed potatoes, egg noodles, or rice. Enjoy!