Preheat the oven to 200°C, fan 180°C, gas 6.
To make the cheddar pastry, sift the flour and salt in a bowl. Dice the butter and lard, then rub into the flour mixture with your fingertips until it resembles fine breadcrumbs. Mix in the cheese, before adding just enough of the beaten egg to bind the mixture together to make a firm dough. Lightly knead the dough, wrap it in cling film and chill until needed.
In a frying pan, heat 1 tbsp of the oil on a medium heat and add the chicken. Cook until it turns white, and is cooked through with no pink remaining. Be sure to wash your hands after handling raw chicken. Add the mushrooms and continue to fry until the chicken is golden-brown and the mushrooms have released all their moisture. Remove from the pan and set aside.
Using the same pan, add the remaining oil and sweat the onion and garlic for 2-3 minutes or until softened. Remove from the heat and set aside with the chicken and mushrooms.
In a medium pan, melt the butter, stir in the flour and cook for about 3 minutes. Stir constantly until a thick smooth paste is formed.
In a bowl, mix the stock, milk and season. Slowly pour the stock mix into the flour mix, whisking constantly on a medium heat until smooth. Reduce the heat and simmer, stirring constantly for about 5 minutes until the sauce has thickened.
Stir in the parsley and chives before pouring the sauce over the chicken and mushroom mix. Stir well, then spoon into the pie dish and leave to cool completely.
On a lightly floured surface, roll out the cheese pastry dough to the thickness of 3mm. Brush the edges of the pie dish with the beaten egg, lay the pastry on top, press down the edges and trim. Brush the top of the pie with some of the beaten egg.
Make two or three slits in the top of the pie to allow steam to escape. Bake in the oven for 20-25 minutes, until the pie turns golden-brown on top. Serve immediately.
Recipe courtesy of Great British Chefs and Nathan Outlaw.