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