Melt a tablespoon of butter in a large, non-stick, flameproof dish or pan over medium heat on the hob. 

Add the chicken and fry for 5 minutes, stirring often until browned. 

Add the onions and fry for a further 5 minutes, stirring often. 

Add the stock, cream, mustard, tarragon and a good pinch of black pepper. 

Simmer for 20 minutes, stirring occasionally. 

Add the ham and peas and stir well. 

Cook for 5 minutes, stirring occasionally. 

Remove from the heat while you prepare the pastry. 

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

Preheat the oven to 180 degrees C (fan-assisted) and grease the bottom and sides of your baking tin with butter.

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

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

Cover gently with the other square of pastry.

Press the edges and the hanging-over bits together by firmly pinching around the edges of the tin with your fingers.

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 golden brown.

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