01 - Pat chicken breasts dry with paper towels. Season both sides generously with salt, black pepper, and dried Italian herbs, pressing the seasoning into the meat to adhere.
02 - Spread chopped onion, minced garlic, and drained sun-dried tomatoes evenly across the bottom of the slow cooker insert.
03 - Arrange seasoned chicken breasts in a single layer over the vegetable mixture, ensuring pieces do not overlap for even cooking.
04 - Pour chicken broth and heavy cream evenly over the chicken and vegetables. Distribute butter pieces across the top.
05 - Cover with lid and cook on low setting for 4 hours or high setting for 2 hours, until chicken reaches internal temperature of 165°F and is fork-tender.
06 - Remove cooked chicken to a plate and cover loosely with foil. Stir grated parmesan cheese into the sauce until melted and smooth, then add baby spinach and stir until wilted, about 2-3 minutes.
07 - Return chicken to the slow cooker, spooning the creamy spinach sauce over each piece. Cook covered for 5-10 minutes longer to heat through. Serve hot, garnished with additional parmesan if desired.