Place the potatoes in a large saucepan and cover in lightly salted water.

Place over medium heat on the hob.

Bring to a simmer and cook for 10 minutes.

Drain and leave to one side until needed.

Melt the butter in a large flameproof dish or pan over medium heat.

Add the leeks and onion and fry for 10 minutes until soft, stirring often.

Sprinkle in the flour and stir well.

Add the cream, stirring in a splash at a time, so there are no lumps of flour.

Stir in the Dijon mustard, grated cheese and a pinch of salt and pepper.

Bring to a simmer and cook for 5-10 minutes until the cheese has melted and the sauce has thickened. Stir occasionally.

Add the drained potatoes and stir.

Remove from the heat and leave to cool.

Roll out the pastry into two squares on a lightly floured surface. One 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. 

Preheat the oven to 180 degrees C (fan-assisted). 

Grease your baking tin or pie dish with butter or margarine.

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

Spoon in the cooled pie filling, filling as much as possible.  

Cover with the other square of pastry. Press the edges together by firmly pinching around the tin.

Trim away any excess.

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

Use a knife to stab two holes into the middle of the pastry so that the steam can escape as it cooks. 

Place into the oven and cook for 25-30 minutes until golden brown. 

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