Carefully pour in the cider—it will boil vigorously—and let it simmer for 4 to 8 minutes, until reduced by about half.
Add the stock and bring to a boil.
Transfer the pan to the oven.
Roast for 10 to 15 minutes, until the chicken thighs reach 165°F on an instant-read thermometer, or their juices run clear when the meat is pierced with a knife. The sauce will be thin, the consistency of pan juice rather than a thick gravy.