Place the raw chicken pieces at the base of your crock pot.
Add the finely chopped onion, minced garlic, Italian seasoning, crushed red pepper flakes if using, and the can of crushed tomatoes on top.
Pour the chicken broth around the ingredients to cover them evenly.
Cover the crock pot and cook on a low setting for 6 to 7 hours, or switch to high for 3 to 4 hours if short on time.
Carefully remove the chicken and shred it with two forks, then return the shredded chicken into the crock pot.
Stir in the heavy cream thoroughly to combine.
If you want a thicker soup, mix cornstarch with cold water to create a slurry and stir it into the crock pot.
Slowly add the freshly grated Parmesan cheese, stirring constantly until the cheese has melted smoothly into the soup.
Leave the crock pot uncovered and let the soup cook on low for an additional 20 to 30 minutes to let flavors blend and thicken.
Season the soup with salt and pepper according to your taste, then sprinkle chopped fresh parsley over the top before serving.