Combine the ground beef, ground pork, chopped rosemary, thyme, sage, Parmesan cheese, egg, lemon zest, salt, and pepper.
Roll the mixture into ½-inch balls, and place them on a large baking sheet. Bake the meatballs for about 10 minutes until just browned.
While the meatballs cook, heat the olive oil in a pot over medium. Cut ½ the lemon into thin slices, and brown them on each side. Toss in the garlic, and cook for 1 minute, until fragrant.
Pour the chicken stock, lemon juice for the other ½ of lemon, Parmesan rind, and sprigs of rosemary into the pan, and bring the liquid to a boil.
Add the pasta and cooked meatballs, and cook the pasta until it's al dente.
Lower the heat so the soup is simmering, then remove the parmesan rind. Season the soup with additional salt and pepper, if desired.
Stir the spinach into the soup. Once the spinach wilts, remove the soup from the heat. Sprinkle the soup with additional Parmesan, if desired.