Season the chicken with the salt and black pepper and put it in a 6-quart slow cooker.
Add the artichokes, onion, garlic and broth, cover and cook on low until the thighs are tender, 5 to 5 1/2 hours.
Move the chicken to a bowl, cut it into bite-size pieces and skim the cooking liquid.
Turn the cooker to high and whisk in the softened cream cheese until smooth, then add the Parmesan.
Stir in the spinach and chicken and cook until the spinach wilts, about 8 minutes.
Boil the pasta shells until just tender and drain them well.
Fold the hot shells through the chicken sauce and serve immediately.