Beat the eggs, parmigiano cheese, lemon rind, and salt in a small bowl.
Bring the broth to a fast boil. Pour the egg mixture slowly into the boiling broth and mix it quickly with a fork to break the egg in small lumps.
Serve hot in soup bowls with a pinch of freshly grated nutmeg (optional).
Pass around the table a small bowl of grated parmigiano cheese for the guests to add on the soup.