Melt butter over medium-high heat in a nonstick pan. Season chicken with salt and peppers. Add chicken to pan and cook until browned on both sides.
Place chicken in a lightly greased 6-quart slow cooker.
In a medium bowl mix together alfredo sauce, sun-dried tomatoes, Parmesan cheese, Italian seasoning, and red pepper flakes.
Pour mixture over chicken. Cover slow cooker and cook on LOW for 3 to 4 hours.
Open slow cooker, mix in spinach. Cover and cook 5 more minutes. Serve.
Notes
I recommend cooking chicken breasts on the LOW setting rather than HIGH for best texture.Leftovers will keep in an airtight container in the refrigerator for 4 days.