Spritz the crock pot with non-stick cooking spray.
Pour frozen meatballs into the slow cooker. Top with the sliced onions.
Add the French onion soup and beef broth in a large mixing bowl. Sprinkle in the two gravy packets and whisk until smooth.
Pour the gravy mixture over the meatballs and onions. Using a large spoon, mix until combined.
Cover and cook on low for 5 ½ hours.
Add the ½ cup of heavy cream. Stir and replace lid. Cook for an additional 30 minutes.
Serve over mashed potatoes or noodles, enjoy!
