Preheat the oven to 375 degrees F and butter a 3-quart baking dish. Melt 2 tablespoons butter in a large skillet over medium heat. Add the sausage and cook, breaking up the meat with a wooden spoon, until browned and cooked through, 6 to 8 minutes. Remove with a slotted spoon and transfer to a plate.
Melt the remaining 6 tablespoons butter in the skillet, then add the onion, celery, apples, sage, thyme, 1 teaspoon salt and a few grinds of pepper. Cook, stirring, until the vegetables are tender, about 5 minutes. Add the cider. Bring to a boil, scraping up any browned bits, and cook until reduced by half, about 5 minutes. Add the broth and return to a boil, then remove from the heat.
Whisk the eggs and parsley in a large bowl. Add the bread, sausage and vegetable-broth mixture and stir until combined. Transfer to the prepared baking dish; sprinkle with the walnuts and drizzle with the turkey drippings (or dot with butter).
Cover the dish with foil and bake 30 minutes, then uncover and bake until golden, about 30 more minutes.
You can assemble this stuffing the day before - keep in the fridge until baking.