Put the raw chicken pieces into the base of your crock pot.
Add the chopped onion, minced garlic, Italian seasoning, crushed red pepper flakes if using, and crushed tomatoes on top.
Pour the chicken broth evenly over all the ingredients.
Cover the crock pot and let it cook on low heat for about 6 to 7 hours, or on high heat for 3 to 4 hours.
Take the chicken out and shred it finely using two forks, then return the shredded chicken back into the crock pot.
Stir in the heavy cream thoroughly.
If you'd like the soup thicker, mix cornstarch with cold water, add this slurry gradually, and stir well.
Slowly incorporate the grated Parmesan cheese, stirring continuously until it has melted into the soup.
Allow the soup to cook uncovered on low for another 20 to 30 minutes to thicken and blend the flavors.
Season with salt and pepper according to your taste and garnish with freshly chopped parsley before serving.