Place chicken thighs in the slow cooker. Season with Italian seasoning, salt, and pepper. Sprinkle the sun-dried tomatoes on top.
Heat the oil from the sun-dried tomatoes and butter in a saucepan over medium heat. Add garlic and sauté for 60 seconds.
Add chicken stock and heavy cream. Simmer gently for 10 minutes until slightly thickened. Whisk in the grated cheese 1/4 cup at a time, then season to taste.
Pour the sauce over the chicken in the slow cooker.
Cook on high for 3-4 hours or low for 6-7 hours.
Remove chicken, set aside, and turn the slow cooker to high. Stir spinach into the sauce until wilted.
Return chicken to the slow cooker or plate it and spoon the sauce over.