In a large sauce pot cook onions, covered, in butter about 20 minutes or until tender.
Add condensed beef broth, Worcestershire sauce, salt, and a dash of pepper; bring to boiling.
Place slice of cheese on toasted bread; place under broiler until cheese is lightly browned.
Ladle soup into bowls and float bread atop. (Or, place a bread slice on soup in each broiler-proof soup bowl; cover with cheese. Broil until cheese is lightly browned.)
