To make the meatballs: Stir the first 6 ingredients in a large bowl to blend. Stir in the cheese, beef and pork. Using 1 1/2 teaspoons for each, shape the meat mixture into 1-inch-diameter meatballs. Place on a baking sheet.
Meanwhile, bring a saucepan of salted water to a boil. Add pasta, cook to al dente. Drain and reserve.
To make the soup: Bring the broth to a boil in a large pot over medium-high heat. Add the meatballs and spinach and simmer until the meatballs are cooked through and the spinach is tender, about 8 minutes. Season the soup to taste with salt and pepper.
Spoon the pasta into each bowl. Ladle the soup into bowls and serve. Finish soup with parmesan cheese if desired.