01 - Pat the chicken breasts dry and season both sides evenly with salt, black pepper, and dried Italian herbs.
02 - Place the finely chopped onion, minced garlic, and chopped sun-dried tomatoes in the bottom of the slow cooker. Arrange the seasoned chicken breasts on top of the vegetable mixture.
03 - Pour the chicken broth and heavy cream over the chicken. Dot the top 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 to an internal temperature of 165°F.
05 - Carefully remove the chicken breasts and set aside. Stir the grated Parmesan cheese and fresh baby spinach into the sauce in the slow cooker. Allow the spinach to wilt, about 5 minutes, stirring gently.
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 generously over each chicken breast. Garnish with extra Parmesan cheese if desired.