Place chicken breasts in the bottom of the Crock Pot.
Add diced onions and minced garlic on top of the chicken.
Pour in chicken broth and diced tomatoes, then stir gently to combine.
Sprinkle Italian seasoning over the mixture, and season with salt and pepper to taste.
Cover and cook on low for 6 hours, or until chicken is tender and fully cooked.
Remove chicken breasts and shred with two forks, then return shredded chicken to the soup.
Stir in heavy cream and grated Parmesan cheese until well combined.
Add tortellini and fresh spinach, then cook on high for 30 minutes until tortellini is heated through.
Serve hot, garnished with fresh basil if desired.