Peel and chop the onion and fry gently until it becomes limp. Take a shallow baking dish about 8" x 5" (20cm x 12.5cm) and grease well with butter. Press the sausage meat into the bottom of the dish then cover with the cooked onion. Top this with the mashed potato and sprinkle the grated cheese on top. Bake for 30 minutes in a preheated moderate oven (Mk 5 - 375ºF - 190ºC). Serve hot either on its own or with a few fresh greens.