To make the soup: Bring the broth to a boil in a large pot over medium-high heat. Add the meatballs and curly endive and simmer until the meatballs are cooked through and the curly endive is tender, about 8 minutes. Whisk the eggs and cheese in a medium bowl to blend. Stir the soup in a circular motion. Gradually drizzle the egg mixture into the moving broth, stirring gently with a fork to form thin strands of egg, about 1 minute. Season the soup to taste with salt and pepper.
Ladle the soup into bowls and serve. Finish soup with Parmesan cheese if desired.