Prepare the Slow Cooker: Lightly grease the insert of your slow cooker with nonstick spray or a small amount of olive oil.
Season the Chicken: Season the chicken breasts or thighs evenly with salt, black pepper, and Italian seasoning. Place them at the bottom of the slow cooker.
Make the Sauce: In a medium bowl, whisk together the heavy cream, grated Parmesan cheese, and minced garlic until well combined.
Add Butter: Distribute the small butter pieces evenly over the seasoned chicken in the slow cooker.
Pour the Sauce: Pour the prepared creamy garlic parmesan sauce over the chicken, ensuring every piece is well coated.
Cook: Cover the slow cooker with its lid and cook on LOW for 6–8 hours or on HIGH for 3–4 hours. Check that the chicken reaches an internal temperature of 165°F (74°C).
Add Spinach (Optional): If you are incorporating spinach, add it to the slow cooker during the last 30 minutes of cooking. Stir gently to allow it to wilt in the hot sauce.
Serve: Once fully cooked, shred or slice the chicken and serve hot over pasta, mashed potatoes, or rice. Spoon extra velvety sauce from the crockpot right on top!