Preheat the oven to 200°C fan
In a deep cast iron pan, fry the pancetta in a little olive oil over a medium heat until browned
Add the shredded Brussels sprouts and sliced leek with a pinch of salt and sauté until softened
Stir through the flour and cook it out for a minute
Slowly whisk in 500ml cold milk until you have a smooth sauce
Crumble in the stock cube, then add the mustard, sage, chestnuts and cranberries
Bring everything to a gentle simmer and cook for 5-10 minutes until the sauce has thickened, loosening with a splash more milk if needed
Fold through the shredded turkey or chicken, then take off the heat and leave to cool for a few minutes
Roll out the puff pastry and use the top of your pie dish as a guide to cut out a lid
Tip the filling into your pie dish, lay the pastry over the top and crimp the edges
Cut two small air vents in the middle, brush all over with beaten egg and bake for about 30 minutes, until the pastry is well risen and deep golden
Serve hot, straight from the pie dish to the table
