Heat the broth: In a medium-sized pot, bring the beef broth to a gentle simmer over medium heat. You want it hot but not boiling aggressively, as this will help create delicate egg ribbons rather than breaking them into small pieces.
Prepare the egg mixture: In a medium bowl, whisk the eggs thoroughly. Once the eggs are well beaten, stir in the grated Parmesan cheese.
Temper the eggs (optional but recommended): To prevent the eggs from cooking too quickly and forming clumps, temper them by slowly adding a few tablespoons of hot broth into the egg mixture while stirring continuously. This will gently warm up the eggs before they hit the pot.
Add the eggs to the broth: Reduce the heat to low. Slowly pour the egg mixture into the simmering broth in a thin stream while stirring the soup gently in one direction. This will help form delicate egg ribbons instead of clumps. Continue stirring for about 30 seconds to ensure even distribution.
Incorporate the spinach: Add the chopped spinach to the soup and let it cook for about 2 minutes, just until wilted.
Season to taste: Add salt and black pepper as needed. Keep in mind that Parmesan cheese adds natural saltiness, so taste before adding extra salt.
Serve immediately: Ladle the soup into bowls and, if desired, top with a little extra Parmesan cheese for extra flavor. Serve hot and enjoy!