Arrange the chicken breasts in the bottom of the slow cooker.
Add the chopped onion and minced garlic over the chicken.
Pour in the chicken broth and diced tomatoes, then stir lightly to distribute the ingredients.
Season with Italian seasoning, salt, and black pepper.
Cover and cook on low for 6 hours, or until the chicken is fully cooked and easy to pull apart.
Lift the chicken out of the slow cooker and shred it with two forks. Place the shredded chicken back into the soup.
Stir in the heavy cream and Parmesan cheese until the broth turns creamy and smooth.
Add the tortellini and spinach. Switch the slow cooker to high and cook for about 30 minutes, until the tortellini is tender and the spinach has wilted.
Ladle into bowls and top with fresh basil before serving, if desired.