Place the chicken breasts or thighs into your slow cooker. Season them lightly with salt and pepper.
In a skillet over medium heat, warm the olive oil. Add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant. Transfer the onion and garlic mixture to the slow cooker, scattering it over the chicken.
Pour the undrained diced tomatoes and chicken broth into the slow cooker. Add the chopped sun-dried tomatoes, Italian seasoning, salt, and pepper. Stir gently to combine.
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 and the size of the chicken pieces.
About 15-20 minutes before serving, remove the chicken from the slow cooker and place it on a cutting board. Shred or slice the chicken.
Stir the heavy cream and fresh spinach into the slow cooker liquid. Cook on HIGH for another 10-15 minutes, or until the spinach has wilted and the sauce has thickened slightly.
Return the shredded or sliced chicken to the slow cooker and stir to coat it in the creamy sauce. If using, stir in the Parmesan cheese.
Serve hot and enjoy the incredible flavor!