Simmer the broth: In a 2-quart pot or saucepan over medium heat, bring the broth to a simmer.
Whisk the eggs: Whisk together the eggs and ¾ cup of the Parmigiano cheese until well combined, save the remaining ¼ cup of cheese to sprinkle on top of the soup just before serving. If using nutmeg, grate in a small amount—about a pinch worth—and whisk to combine. Set aside
Add the spinach and lemon zest to the broth: Stir the chopped spinach into the simmering chicken broth and cook until the spinach is heated through and the broth is simmering once more, about 5 minutes. Don’t be alarmed at the amount of spinach. It will wilt down to only about 1 cup in the soup. Stir in the lemon zest, if using.
Add the eggs: Slowly pour the egg mixture into the soup. Gently stir once or twice with a wooden spoon or a fork. Simmer, uncovered, for a couple of minutes without stirring, until the eggs are fluffy and set. They will float on top of the broth in a soft mass. Gently break up the eggs into soft clumps.
Serve: Ladle the soup into warmed bowls and sprinkle the remaining Parmigiano-Reggiano on top. Serve hot.
Make sure to use a high quality chicken broth: it’s a major flavor component of this dish.
For large fluffy egg curds, pour them into the simmering broth, wait, allow them to set, and then stir. For smaller curds stir as you’re pouring the eggs in.