Melt butter in a large saucepan.   Add onions, garlic and sugar and cook, stirring, for about 20 minutes or until onions have caramelized. 

     In a small bowl, mix the flour with enough white wine to make a smooth paste, then stir into onion mixture.     

     Cook 2 minutes, then add beef broth and remaining wine.  Cover and simmer for 30 minutes.

     Preheat broiler.  Ladle hot soup into 6 ovenproof bowls; top each bowl with a piece of French bread.  Sprinkle evenly with cheese.  Broil 5 minutes or until cheese is golden brown.