Place sliced onions in a 6 quart slow cooker.
Stir in melted butter, sugar, salt, and pepper.
Top with thyme and bay leaf, COVER and cook on LOW for about 8 hours, or until onions are browned and caramelized. Stir half way through.
Once onions are caramelized, stir in the beef broth and wine.
Taste for salt and pepper and season accordingly.
Cover and cook on LOW for 4 hours, or on HIGH for 2 hours, or until heated through.
Remove and discard thyme and bay leaf.
Turn the oven-broiler on High.
Ladle soup into ovenproof bowls.
Top the bowls with a slice of french bread and cheese.
Broil in oven until cheese is melted, about 2 to 3 minutes.