Heat the oven to 425°F (220°C) with a rack in the middle. Have a roasting pan (or large ovenproof skillet) ready.
In the roasting pan, toss the apples, onion, and garlic with the olive oil. Nestle in the thyme sprigs and sage leaves.
Pat the chicken dry, tuck the wing tips under, and set it breast-side up on top of the apples and onions. Brush all over with 2 tablespoons of the melted butter; reserve the remaining 1 tablespoon for finishing the sauce later. Season the chicken and the apples and onion with the kosher salt and black pepper. Pour 0.5 cups of the dry hard cider into the pan and reserve 0.5 cups for the sauce.
Roast for 20 minutes, then reduce the oven temperature to 375°F (190°C) and continue roasting 35–45 minutes more, until the thickest part of the thigh registers 165°F and the apples are tender and caramelized at the edges.
Transfer the chicken to a board and rest 15 minutes. Toss the roasted apples and onions in the pan to coat with juices and keep warm.
Set the roasting pan over medium heat and spoon off excess fat if needed. Add the Calvados and simmer 1–2 minutes, scraping up brown bits, until nearly evaporated. Add the reserved 0.5 cups dry hard cider and the chicken stock; boil 5–7 minutes until reduced by about half. Whisk in the Dijon mustard and any resting juices from the chicken, then add the heavy cream and simmer 2–3 minutes more until lightly thickened; remove from heat and swirl in the reserved 1 tablespoon melted butter. Taste and adjust seasoning.
Carve the chicken. Spoon the roasted apples and onions onto plates, top with chicken, and ladle the cider-cream sauce over to serve.