Peel the potatoes and cut them into thin slices. Peel and chop the onion. Place the minced beef and chopped onion into a frying pan with the duck (or goose) fat and fry gently, stirring regularly, for about 5-10 minutes until the meat is well browned. Drain off any fat either using a slotted spoon or pouring the contents of the pan into a sieve (strainer) or colander for a few moments, then returning them to the pan. Add the soup, milk and Worcestershire sauce and stir well.
Simmer the mixture gently for 5 minutes, stirring occasionally, then place it into a 2 pint (1.2 ltr) ovenproof dish. Top the mixture with the potato slices and bake in a preheated moderately hot oven (Mk 6 - 400ºF - 200ºC) for about 30 minutes until the potatoes are tender.