01 - Bring the chicken or vegetable broth to a gentle simmer in a medium saucepan over medium heat.
02 - In a mixing bowl, whisk together the eggs, grated Parmesan, breadcrumbs (if using), parsley, salt, and pepper until thoroughly combined.
03 - Reduce the heat to low. While stirring the simmering broth continuously with a fork or whisk, slowly drizzle the egg mixture in a thin stream. The eggs will cook instantly, forming delicate, wispy ribbons throughout the broth.
04 - Stir in the chopped spinach and cook gently until just wilted, about 1 minute. 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.