Finely chop the mushrooms and cook in the butter with the chopped thyme for 5 minutes or until most of the liquid is cooked off. Cool.
Toss the shredded turkey or chicken in the zest and the juice of the lemon, and the thyme.
Roll out the pastry to about ¼ inch thick. Spread some cranberry sauce on the pastry, keeping away from the edges. Now lay half the turkey on top. Now cover with the leek, the rest of the turkey and then fold and seal the pastry pie.
Brush the pastry with a beaten egg and cook in a hot oven 200c/gas mark 6 until brown, which should take around 25 minutes.