Slice onions and lay them in the bottom of a 6 Qt. slow cooker. Sprinkle with salt, pepper, and sugar. Dot with butter. Set to cook on LOW for 9 hours. **Optional method is to cook onions in a skillet until caramelized, approximately 15-20 minutes**
In a bowl, mix ground beef, bread crumbs, milk, egg, and onion soup mix. Shape into a loaf.
Pour cooking sherry and Worcestershire sauce over the onions. Place the meatloaf on top of the onions. Cook on HIGH for 3 hours or until internal temperature reaches 160°F.
Top meatloaf with cheese slices and cook for an additional 20-30 minutes until cheese is melted.
Serve sliced with caramelized onions and juice from the cooker. Garnish with fresh thyme.