Slice the onions and place them in the bottom of a 6 Qt. slow cooker. Add the salt, pepper, and sugar on top, then dot with butter. Cook on LOW for 9 hours, or caramelize in a skillet for 15-20 minutes.
In a mixing bowl, combine the ground beef, Italian breadcrumbs, milk, egg, and French onion soup mix. Form into a loaf shape.
Pour cooking sherry and Worcestershire sauce over the onions. Carefully place the meatloaf on top.
Cook on HIGH for 3 hours, or until the internal temperature reaches 160°F.
Top the meatloaf with Swiss cheese slices and let cook for another 20-30 minutes until the cheese melts.
Slice and serve with caramelized onions and juices from the cooker, garnished with fresh thyme.