In a separate bowl, beat the egg with a fork and continue beating as you add the cheese and bread crumbs. When they are well beaten, add 1/2 cup of the hot stock.
Bring the remaining stock to nearly a boil, scrape in the egg mixture, and whisk it with a fork for 3 or 4 minutes. When you stop beating and let the soup come just barely to a boil. It will break into "rags," hence the name.
Serve immediately, with an extra sprinkling of Parmesan, topped by garnish of your choice.