Melt butter in heavy large saucepan over medium heat. . Add onions, sugar and pinch of salt. Cover and cook until onions are very soft, stirring occasionally, about 20 min. Increase heat to high and sauté uncovered until onions are golden brown. About 10 minutes. Add flour and stir 2 minutes. Mix chicken and beef broth, wine, brandy and mustard. Bring soup to boil, stirring frequently. Reduce heat to medium low cover and simmer 10 minutes. Season to taste with salt and pepper. (Can be prepared 1 day ahead. Cover and refrigerate Re-warm before continuing.) Pre heat broiler. Ladle soup into deep broiler proof bowls. Top soup with bread slices and cheese. Broil until cheese melts and begins to brown.