In large saucepan or Dutch oven, cook onion in butter until tender, but not brown, about 20 minutes.
Add beef broth, sherry, Worcestershire sauce and pepper, bring to a boil.
Pour into 6 individual oven-proof bowls.
Float a slice of bread in each bowl (rub toast with a cut clove of garlic, if desired) sprinkling with Parmesan cheese and top with Gruyere or Swiss cheese slice.
Bake in 375° oven for 15 minutes, or boil till cheese is bubbly, about 5 minutes.
This soup freezes well.