Melt the butter in a saucepan and gently saute the leek for 5 minutes to soften it, taking care not to colour.
Cut up all your leftover turkey and vegetables into bite sized pieces, place in a large bowl and mix together with the sauted leek, gravy and creme fraiche. [How much gravy and creme fraiche you use will depend on the amount of leftover meat and vegetables you have.]
Spoon into an oven-proof dish and cover with a layer of puff pastry. Pierce the pastry lid a couple of times and brush with beaten egg.
Place in an oven at 180Fan for 40 minutes or until the pastry is puffed and golden.
When using up leftover vegetables avoid adding things like pickled red cabbage as the vinegar will sour the pie and turn everything a rather unappetising pink colour. Instead heat up the cabbage and serve on the side.As well a the turkey you can add leftover ham, duck or goose to the pie.If the gravy needs a lift add some fresh thyme leaves to it.