Heat the oven (375F/190C/Gas Mark 5-6). Grease an ovenproof dish.
Slice the cooked potatoes. Peel and chop the onion. De-rind the bacon. Prick the sausages.
Heat the oil in a frying pan, and fry the sausages, bacon and onion gently for 5 minutes, turning frequently.
Place the potato slices in the dish. Arrange the onions, sausages, and bacon on top. Beat the egg with the milk, salt and pepper in a small basin, using a whisk or fork. Pour the egg mixture over the top, and bake in a hot oven for about 15 minutes, until the egg mixture is set.