Place the chicken breasts or thighs into the bottom of your slow cooker.
In a medium bowl, whisk together the diced tomatoes, chicken broth, chopped sun-dried tomatoes, heavy cream, Italian seasoning, garlic powder, and black pepper. Season with salt to your preference.
Pour the liquid mixture evenly over the chicken in the slow cooker.
Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours, until the chicken is cooked through and tender. The exact time will depend on your slow cooker's model and the size of the chicken pieces.
About 15-20 minutes before serving, stir in the fresh spinach and grated Parmesan cheese. Allow the spinach to wilt into the sauce.
Shred or slice the chicken directly in the slow cooker, or remove it to shred/slice and then return it to the sauce. Stir to combine.
Taste and adjust seasonings if necessary.