Place the diced chicken breasts at the bottom of the crockpot.
Add the chopped onion, minced garlic, diced tomatoes (with juices), chicken broth, Italian seasoning, and red pepper flakes (if using) into the crockpot.
Stir the mixture until everything is well combined.
Cover the crockpot and cook on low for 6-7 hours or on high for 4 hours, until the chicken is cooked through and tender.
Once the cooking time is up, shred the chicken using two forks while still in the crockpot.
Stir in the heavy cream, grated Parmesan cheese, and half of the mozzarella cheese into the soup. Mix well until the cheese has melted and the soup is creamy.
Season with salt and pepper to taste.
Serve the soup hot, garnished with the remaining mozzarella cheese and fresh basil leaves on top.
Notes
Feel free to adjust the level of heat by modifying the red pepper flakes.