1. Melt butter in large saucepan that holds at least 4 quarts. Add onions; cook over medium heat 12 minutes, or until tender and golden. Stir often.
2. Add sugar and cook, stirring, for 1 minute. Add broth; cover and bring to a boil. Reduce heat; simmer 12 minutes. If desired, add brandy; cook 2 minutes longer. Season with salt and pepper.
3. To serve, ladle soup into bowl; float toast on soup. Sprinkle with cheese.