1. For the pastry, sift the flour into a bowl and add the salt. Dice the butter and lard, then combine into the flour mixture with your fingertips until it resembles fine breadcrumbs.
2. Mix in cheddar, then add just enough of the egg to bring together as a firm dough. Knead lightly and wrap the dough in cling film. Chill until required.
3. Preheat the oven to 200°C/gas mark 6.
4. Heat the olive oil in a large pan, then add the chicken and fry until the chicken begins to turn white. Add the mushrooms and continue to fry until the chicken is golden-brown. Remove the mixture from the pan and set aside.
5. Add the onion and garlic to the same pan and fry for 2-3 minutes or until softened. Remove from the heat and set aside with the chicken and mushrooms.
6. Melt the butter in a large, clean saucepan. Add the flour and cook for about 3 minutes, stirring constantly until it has formed a thick, smooth paste.
7. In a bowl, mix the stock, milk, white pepper and salt together. Pour the liquid slowly into the flour mixture a bit at a time, stirring constantly until smooth. Simmer over a gentle heat, stirring constantly for about 5 minutes, or until the sauce has thickened.
8. Stir in the chives and parsley, then pour the sauce over the chicken, mushrooms, onions and garlic. Mix well, then spoon into a pie dish and leave until completely cool.
9. Roll the pastry dough out on a lightly-floured surface until it is the thickness of a pound coin (around 3mm). Brush the edges of the pie dish with beaten egg, lay the pastry on top, press down the edges and trim. Brush the top of the pastry with beaten egg.
10. Make two or three slits in the top of the pie to allow steam to escape, then bake the pie in the oven for 20-25 minutes, or until golden-brown on top.
Recipe courtesy of www.greatbritishchefs.com