01 - Pour the chicken or vegetable broth into a medium saucepan and set it over medium heat. Bring to a gentle simmer, avoiding a full rolling boil.
02 - In a mixing bowl, whisk together the eggs, grated Parmesan, breadcrumbs (if using), chopped parsley, salt, and pepper until smooth and well combined.
03 - Reduce the broth heat to low. While stirring the simmering broth continuously with a fork or whisk, slowly drizzle in the egg mixture in a thin stream. The eggs will cook on contact, forming delicate, wispy ribbons throughout the soup.
04 - Stir in the chopped spinach, if using, and cook gently for about 1 minute until just wilted. Taste the soup and adjust the salt and pepper as needed.
05 - Ladle the hot soup into bowls and finish with an extra sprinkle of grated Parmesan cheese if desired.