Heat a large flameproof dish or pan over medium heat on the hob.

Add the butter and melt.

Add the onions and fry for 5 minutes, stirring often until they are starting to soften.

Stir in the mushrooms.

Cook for a further 5 minutes, stirring often, until the onions are translucent and the mushrooms have shrunk.

Add the leftover turkey and the diced potatoes and stir well.

Pour in the stock and cream and stir.

Add the mustard, tarragon and freshly ground black pepper. Stir together until smooth.

Bring to a simmer and reduce the heat if you need to.

Simmer for 30 minutes, stirring occasionally, until the potatoes have softened and the sauce has reduced.

Remove the cooking pan from the heat and leave the filling to cool slightly.

Liberally grease your tin or dish with butter and preheat the oven to 180 degrees C (fan-assisted).

On a lightly floured surface, roll out the pastry into two squares. One should be big enough to fill the bottom and sides of the tin with a little hanging over it. The other should be big enough to cover the top.

Place the larger pastry square into the tin and press gently into the bottom and sides, leaving a little hanging over.

Spoon in the cooled pie filling, filling to the top if possible.

Place the other square of pastry on top.

Press the edges and the overhang together by firmly pinching around the edges of the tin with your fingers.

Carefully trim away any excess.

Beat an egg in a small bowl.

Use a pastry brush to brush the beaten egg liberally onto the top of the pie.

Use a knife to stab two holes into the middle of the pastry.

Place into the preheated oven and cook for 30 minutes until the top is golden brown.

Remove from the oven and rest for a few minutes before serving.