Return it to the oven for 45 minutes, at which point the duck will be cooked and tender, and the potatoes incredible soft. Increase the heat at this point to 400° to crisp the potatoes. Keep a careful eye on the duck—you don’t want the skin to burn, so if it looks like it’s browning too much, remove it to a plate and cover it with foil. In all, this should only take about 10 minutes or so.