Place duck, breast side up, on a rack in a roasting pan. Cover breast of the bird with buttered foil and roast in a slow oven (350°-M3) for 1½ to 1¾ hours. Remove foil from time to time and baste duck with pan juices. Remove foil 20 minutes before removing duck from the oven; dredge breast with flour and leave in oven until well browned. Transfer to a hot serving dish; remove trussing threads and strings, and keep duck warm while preparing gravy.