01 - Pat the chicken breasts dry and season both sides evenly with salt, black pepper, and dried Italian herbs.
02 - Spread the chopped onion, minced garlic, and chopped sun-dried tomatoes across the bottom of the slow cooker. Place the seasoned chicken breasts on top of the vegetable layer.
03 - Pour the chicken broth and heavy cream over the chicken. Dot the surface with pieces of unsalted butter.
04 - Cover the slow cooker and cook on low for 4 hours or on high for 2 hours, until the chicken is fork-tender and cooked through.
05 - Carefully remove the chicken breasts from the slow cooker. Stir the grated Parmesan cheese and fresh baby spinach into the sauce, allowing the residual heat to wilt the spinach, about 5 minutes.
06 - Return the chicken breasts to the slow cooker, spoon the creamy sauce over the top, and cook for an additional 5 to 10 minutes until everything is heated through.
07 - Serve hot, spooning the creamy Tuscan sauce over each chicken breast. Garnish with extra Parmesan cheese if desired.