Line a baking sheet with parchment paper and set aside.
Place all meatball ingredients in a medium to large bowl. Mix everything together with your hands until just combined.
Shape the meat mixture into 1/2- to 1-inch balls and place on the prepared baking sheet.
Cover with plastic wrap and refrigerate for at least 1 hour.
Heat the olive oil in a large, heavy-bottomed pot over medium heat. Add the onion, carrots, celery, and garlic. Cook until the onions are translucent.
Add the meatballs and simmer for 7 minutes.
Add the pasta and simmer for 6 more minutes.
Add the spinach and simmer for an additional 3 minutes.
Next, add the shredded chicken and simmer for approximately 30 minutes more. Add salt and pepper to taste.
Note: Rotisserie chicken can be used in lieu of cooking your own chicken breasts.