First make up the pastry by rubbing the flour into the butter till crumbly, then add the cheese, mustard and seasoning together with just enough cold water to make a dough that leaves the bowl clean. Place the dough in a polythene bag and leave to rest in the refrigerator for 20 minutes or so.
Meanwhile melt the teaspoon of butter in a frying pan, swirl it around the pan and then, keeping the heat low, add the leeks and let them gently cook for about 6 minutes and exude some of their juice. After that place the leeks in a sieve, strain off any juices into a bowl and set aside.
Next roll out the pastry and use it to line the prepared quiche tin. Prick the base with a fork, then bake on the centre shelf of the oven for 15 minutes.
While that's happening put the 1 1/2 oz (40g) butter, milk and flour in a saucepan and bring up to the boil, whisking all the time, until you have a smooth thick sauce. Season with salt and pepper and a little nutmeg, then leave the sauce to simmer gently for 5 minutes.
Remove the pastry case from the oven and arrange the leeks over the base, followed by the slices of turkey. Now pour the reserved leek juice in the sauce, add three-quarters of the grated cheese and the beaten egg and mix well. Pour the sauce evenly over the contents of the flan and sprinkle the remaining chesse on top together with a dusting of cayenne. Bake the flan at 375F/190C/Gas 5 on the baking sheet for 20 - 30 minutes or until nicely browned on top.
This recipe is from Delia Smith's Christmas.