Whisk soup, broth, gravy mix, onion powder, garlic powder, thyme, and pepper until smooth.
Arrange chicken in the slow cooker. Dot the top with butter.
Pour the gravy mixture over the chicken, coating evenly.
Cover and cook on Low for 5 hours, until the chicken is very tender.
Transfer chicken to a plate. Shred or slice, then keep warm.
Stir cornstarch and water to make a slurry. Whisk into the slow cooker.
Cover and cook 10–15 minutes, until the gravy thickens. Stir in sour cream.
Return chicken to the gravy and coat. Garnish with parsley and serve.