If fall had an official scent, I swear it would be onions sizzling in butter, apples softening in sage and thyme, and warm sourdough cubes soaking up the most delicious savory broth. This Apple–Chestnut Stuffing is one of those dishes that walks into a room and announces, “Yes, Thanksgiving has officially begun.” Whether you’re hosting dinner this year or simply bringing a dish to impress the in-…