In a large sauce pan melt butter. Stir in onions. Cook covered on medium heat 8-10 minutes until tender and golden - stir occasionally. Add beef broth, dry sherry (if desired), worcestershire sauce and a dash of pepper. Bring to a boil, reduce heat, cover and simmer 10 minutes. Sprinkle toasted bread with cheese. Place under broiler til cheese melts and turns light brown.