Peel potatoes chop into small chunks & cook in a pot until potatoes are cooked than mash with a potato ricer add 4 cubes of butter & ¼ cup of milk mix till smooth than set a side.
If using left over roast lamb chop small. If using lamb mince cook lamb till lamb is cooked. Add chopped onion & garlic & cook a fewer couple of minutes or until soft .Add carrot & celery & cook until veg is soft. Add stock & cook. Bring to the boil than turn down to simmer & cook for 30-45 mins. .Let the mix reduce but if still quite runny thicken with some cornflour & water mixed to a smooth paste.
Put lamb mix into a pie dish and top with the mashed potato and grated cheese. Cook in a 180 C oven for 12-20 minutes or until the topping is golden. Should feed a family of 4-6.