Pat the chicken breasts dry with paper towels. Season them generously on all sides with Italian seasoning, onion powder, red pepper flakes (if using), salt, and pepper.

Lightly grease the insert of your slow cooker (minimum 5-quart size recommended) with nonstick spray or a thin layer of oil. Place the seasoned chicken breasts in the bottom of the slow cooker, arranging them in a single layer as much as possible.

In a medium bowl, whisk together the chicken broth, heavy cream, grated Parmesan cheese, chopped drained sun-dried tomatoes, and minced garlic until well combined.

Pour this creamy sauce mixture evenly over the chicken breasts in the slow cooker.

Secure the lid on the slow cooker. Cook on the LOW heat setting for 5 to 6 hours or on the HIGH heat setting for 3 to 4 hours. Cooking time is complete when the chicken is cooked through and tender (internal temperature should reach 165°F / 74°C). Avoid checking chicken temperature too early or lifting the lid frequently, as this releases heat.

Once the chicken is cooked, turn the slow cooker off or to the "keep warm" setting. Stir the chopped baby spinach and chopped fresh basil (if using) into the sauce in the slow cooker.

Replace the lid for about 5-10 minutes, allowing the residual heat to wilt the spinach completely.

Taste the sauce and adjust seasoning with additional salt and pepper if necessary.

Serve the chicken breasts whole with a generous amount of the creamy sun-dried tomato and spinach sauce spooned over the top.