Cook and stir onions in butter in a covered large saucepan over medium-high heat for about 30 minutes. Stir in beef broth, Worcestershire sauce, A.1. steak sauce, sherry, salt and pepper; place onto a medium-high stove, heat to a boil and cook about 30 to 45 minutes. Remove from heat.
Place well buttered bread on a cookie sheet and place a slice of cheese on top, put into a oven set on broil, 4 to 6 inches away from heat and bake about 1 minute or until bread is toasted and cheese is melted.
Ladle soup into soup bowls and top with bread slices.