Take an 8" (20cm) flan dish and grease well with butter. Use the cooked potato to line the dish up to ½" (1cm) thick. Slice the cooked sausages thinly. Beat the eggs, peel and chop the onion and grate (shred) the cheese.
To make the filling, place the sausages, beaten eggs, milk, grated cheese, chopped onion, mustard and salt and pepper into the potato flan case then cook the whole flan in a preheated moderately hot oven (Mk 6 - 400ºF - 200ºC) for 25-30 minutes until it is firm and golden. Eat hot with green vegetables or cold in a buffet.