Place the sliced onions in the bottom of a 6-quart crock pot. Then, pour frozen meatballs over the top.
Add beef broth, gravy mix, garlic, black pepper, dried thyme, and white wine vinegar in a medium bowl. Whisk until well combined and pour over meatballs.
Cover and cook on high for 2½-3½ hours or low for 5-6 hours.
In a small bowl, whisk cornstarch and water together. Pour over the meatballs and mix until well combined. Then, top with gruyere cheese and cook for ten minutes or until the cheese is melted.
Serve over rice or mashed potatoes, and enjoy!
