Season both sides of the chicken breasts with Italian seasoning, onion powder, kosher salt, and freshly cracked black pepper. Lightly coat the inside of the crockpot with nonstick cooking spray, then arrange the seasoned chicken in a single layer at the bottom. In a separate mixing bowl, combine the chicken broth, heavy cream, minced garlic, chopped sun-dried tomatoes, and grated Parmesan cheese, whisking until well blended. Pour the prepared sauce evenly over the chicken in the crockpot. Cover and cook on high for 3 hours or on low for 5 hours, until the chicken is fully cooked and tender. Just before serving, stir in the chopped spinach, cover again, and let it wilt for a few minutes. Taste and adjust seasoning with additional salt and pepper if needed. Serve the creamy Tuscan chicken over your favorite cooked pasta and enjoy!