Season the chicken: Pat the thighs dry and season with the salt and pepper.
Sear skin-side down: Heat the oil in a large ovenproof skillet over medium-high heat and sear the thighs skin-side down for 5 minutes, until deeply golden.
Finish the sear: Turn and sear 5 minutes more, then transfer the thighs to a plate.
Soften the apples and onions: Cook the onion and apples in the same skillet for 5 minutes, until just softened at the edges, stirring in the garlic and thyme for the last minute.
Build the braise: Pour in the cider and Dijon, scraping up the browned bits, and bring to a simmer.
Braise until tender: Return the chicken to the pan, cover, and simmer on low for 30 minutes, until the chicken reaches 165°F at the thickest part.
Finish the sauce: Stir in the vinegar, scatter with parsley if using, and serve with the apples, onions, and sauce spooned over the top.