Saute onions in butter until tender
Add beef broth, sugar, wine, Worcestershire sauce and salt and pepper
Bring to a boil, then reduce heat and cook over medium heat for 15 - 20 minutes
Pour into ovenproof bowls and add bread cubes to each bowl
Place 1 slice of cheese over each bowl of soup
Bake in the oven for 10 minutes at 350 degrees until cheese is melted and golden brown