Preheat the oven to 200°C/gas mark 6.
Heat the olive oil in a large saucepan over a medium heat, before adding the leeks. Allow to cook for 1 minute.
Add the mushroom and cook for a further minute.
Next, add the chicken stock and double cream to the pan and stir. Bring to the boil and reduce the liquid until it starts to thicken.
Add the cooked chicken and tarragon, and season well with salt and pepper. Stir well.
Once the chicken has heated through, set aside from the hob and spoon the mixture into your pie plate. Fill to the top.
Cut you pastry to the size of the dish, and lay it over the filling. Cut around the edge, removing any excess, before crimping the pastry at the edges to make sure your pie is well sealed.
Glaze the pastry by brushing with a beaten egg.
Bake your pie in the oven for 30 minutes, or until the pastry is cooked and is a golden brown colour.
Remove from the oven, slice and serve.
Recipe courtesy of Great British Chefs. Visit their site for more great pie recipes.