Prick the skin of the duck in a few places with a fork. Put the duck into a large roasting dish. Rub the skin with the butter and season with salt and pepper. Put into the hot oven, breast side down, and
Remove from the oven, turn the duck over and pour over the Marsala. Add the onions, potatoes, bay leaves, olives, sage, rosemary and garlic. Mix them through with a wooden spoon. Add
Return the dish to the oven for another 1¼ hours or so, turning the duck over again halfway through cooking, and shuffle the potatoes with a wooden spoon.
There should only be a little liquid left with a crispy duck, which should be well cooked throughout. Remove from the oven and check the potatoes to see if they need salt.
Cut up the duck with poultry shears, carve some of the meat and serve hot, together with the potatoes, onions and pan juices.