For the Meatballs:

1. Preheat the oven to 425°F. Line a baking sheet with aluminum foil.
2. In a large bowl, gently fold all of the ingredients together with your hands until just incorporated. Do not overmix.
3. Roll the mixture into individual teaspoon-sized balls, placing them on the prepared baking sheet about 1/2 inch apart from one another. The mixture will yield about 60-65 meatballs.
4. Transfer the baking sheet to the oven and bake for about 10-12 minutes, until the meatballs are just starting to brown but are still slightly underdone in the center. Remove from the oven and set aside on the pan as you prepare the soup.

For the Soup:

1. In a large stockpot, heat the olive oil on high heat. Add the onion, carrot, and celery and saute until lightly browned, about 5 minutes. Add the garlic and saute for an additional minute.
2. Pour in the chicken stock and bring to a boil. Add the meatballs and shredded chicken, if using, and reduce the heat to a simmer. Let the soup simmer for 15-20 minutes, stirring occasionally.
3. Mix the dry pasta into the simmering soup, and cook until al dente according to the package instructions (usually 6-10 minutes), stirring occasionally.
4. Remove the pot from the heat and add the spinach, stirring it into the soup for about a minute until wilted.
5. Season the soup to taste with salt, and serve with grated Parmigiano Reggiano and freshly cracked black pepper.