Pat the chicken thighs dry and season both sides with salt and pepper; heat the skillet over medium-high until hot and add the oil, then brown thighs skin-side down without moving for 5–6 minutes until deep golden, flip and brown the other side 3 minutes, then transfer to a plate and reserve the fond.

Lower heat to medium, add butter and sliced onions to the pan, spread into an even layer and cook undisturbed for a few minutes, stir every 3–4 minutes and cook 15–20 minutes until soft and golden at the edges, adding 1–2 tablespoons water if sticking to prevent burning.

Pour in apple cider and scrape up the fond, add chicken stock and apple cider vinegar, return chicken to the pan skin-side up so it sits halfway in the liquid, bring to a gentle simmer then reduce heat to low, cover partially and simmer gently 22–28 minutes, checking internal temperature starting at 20 minutes until thighs reach 165°F.

Remove chicken to a plate and tent with foil to rest 5–8 minutes, increase heat and reduce the braising liquid uncovered 4–6 minutes until it coats the back of a spoon, taste and adjust seasoning, spoon onions and sauce over chicken and garnish with parsley if desired.