In a 4 quart saucepan over medium heat in hot butter, cook onions and sugar for 10 minutes.
Stir in flour until well blended with the onions and the pan juices.
Add water, wine, and undiluted beef broth; heat to boiling. Reduce heat to low; cover and simmer 10 minutes.
Cut four 1 inch thick slices of bread from loaf. Toast in 325F oven just until lightly browned, about 10 minutes.
Ladle soup into 4 oven safe bowls and place 1 slice toasted bread on surface of soup in each bowl.
Fold Swiss cheese slices and fit onto toasted bread slices in soup.
Place soup bowls in pan for easier handling. Bake in 425F oven for 10 minutes or just until cheese is melted.