Heat oven to 400° F. Arrange four 10-ounce oven-safe soup bowls on a baking sheet.
Divide the broth and onions among bowls. Add 1 tablespoon of sherry to each. Float a slice of bread in each bowl and top with a slice of cheese.
Bake 12 to 15 minutes or until the cheese is melted and the soup is bubbling.