Combine Ingredients Place the frozen meatballs in the slow cooker insert. In a medium bowl, whisk together the condensed French onion soup, dry onion soup mix, beef broth, Worcestershire sauce, and dried thyme. Pour this mixture over the meatballs in the slow cooker, ensuring they are as covered as possible.

Cook Cover the slow cooker and cook on LOW for 4-6 hours or on HIGH for 2-3 hours, or until the meatballs are heated through and the sauce is hot and bubbly. Stir occasionally if possible.

Add Cheese Just before serving (during the last 15-30 minutes of cooking), sprinkle the shredded Gruyère or Swiss cheese over the top of the meatballs and sauce.

Melt Cheese Cover the slow cooker again and let it cook for another 15-30 minutes, or until the cheese is completely melted and bubbly.

Serve Serve the meatballs hot, spooning the French onion sauce over them. Serve with cooked egg noodles, mashed potatoes, or crusty bread for soaking up the sauce if desired.