Using a large frying pan, slowly saute the onions in butter until limp and glazed. Transfer to crockpot. Add beef stock, white wine, sherry, worcestershire and garlic. Cover. Cook on low (200F) 6 to 8 hours. Place french bread on a baking sheet. Sprinkle with cheese. Place under preheated broiler until lightly toasted. To serve, ladle soup into bowl. Float a slice of toasted french bread on top.