Lots of cooked chicken - basically "Pick" the carcase of a roast chicken and chop the meat.
A similar amount of cooked ham, cubed if possible
Small punnet mushrooms
About 20-25 g butter
250 ml milk
1 heaped tsp flour
A little more butter and/or 1 tbs milk
Chop leek and put in saucepan with butter and mushrooms, which also chop if necessary. Cover and allow to cook on a low heat for 6-7 minutes, until the juices have run.
Meanwhile, peel and steam or boil the potatoes, and mash with the extra butter and milk (and salt and pepper).
Whisk the flour into the milk and pour the result on to the leeks and mushrooms. Bring to the boil, stirring all the time. Now add the chopped meat, and transfer the lot into an oven-proof dish. Top with mashed potato (or you could make your pie with a pastry topping, if you liked), and bake in a moderate oven for about 30 minutes.