Cut the butter and lard into small cubes. Put them in a large glass bowl with the salt and flour. If you are using salted butter, then leave out the salt.
Gently rub the butter with the flour with your fingertips until the mixture resembles fine breadcrumbs.
Add enough of a beaten egg until the mixture comes together into a ball. Knead it briefly until the dough is smooth.
To use a food processor tip the ingredients into the bowl, attach the lid, and pulse the mixture three or four times until it gets to the breadcrumb stage.
Next add the beaten egg and pulse twice.
Turn the processor on full and within a few seconds the mixture should come together into a ball.
Chill the dough for 30 minutes.
Cut the mushrooms, ham and turkey into equal bite sized pieces.
Heat the oil in the frying pan over a medium heat and add the mushrooms. Cook for about 5 minutes until the mushrooms are softened, then set them aside.
In the same pan, add the butter until melted. Tip in the flour and stir together for one minute to cook the flour.
Mix the stock and milk together and gradually pour into the roux mixture, stirring all the time.
As the sauce thickens, keep stirring and cook for a further minute.
Check the seasoning, stir in the tarragon and allow the mixture to cool for a few minutes.
Tip the cubed leftover turkey, ham and cooked mushrooms into the sauce and stir well. Transfer the mixture to the pie dish.
Dust the work surface and rolling pin with a small amount of flour.
Roll out the dough to about half a centimetre thick and lay over the pie dish.
Cut away the excess pastry and flute the pie edges. Roll out the remaining dough to decorate the pie. the dough.
Beat the egg with the water and stick the decorations to the pastry.
Use the remaining egg wash to glaze the pie before baking.
Preheat the oven and bake at 190 C / 375 F / 170 FAN / Gas 5 for about 30 minutes until golden.
Let the pie rest for a few minutes before serving.