Because filling the turkey with stuffing requires a longer roasting time and can increase the risk of food-borne illness, Chef Sam Hayward recommends baking the dressing separately and then enriching it with juices from the turkey’s roasting pan.
Preheat oven to 350º. If you’re using fresh cornbread, lightly toast the cubes about 5 minutes, turning once, until golden. Set aside.
In a large heavy-bottomed pot, cook bacon or pancetta over moderate heat until golden. Add butter, celery root, leek, and shiitakes. Cook, stirring frequently until leeks are tender, about 5 minutes. Add stock and cider, bring to a boil, reduce flame, and simmer until celery root is tender but still intact, about 10 minutes. Remove from flame and add cornbread. Season with salt and pepper to taste, add herbs, and toss gently to moisten. Remove from heat, cover pot, and allow the mixture to steam 10 minutes to develop flavor and aroma. If you want a moister stuffing, add a little more turkey stock and/or cider.
Turn the mixture into a gratin dish or ovenproof ceramic baker and baste the surface generously with turkey drippings. Bake 20 minutes before serving.