To make the pastry, put the flour into a bowl and mix in the salt. Add the butter and lard and rub into the flour with your fingertips until the mixture resembles fine breadcrumbs. Alternatively, do this in a food processor or mixer and then transfer to a bowl.
Using a table knife, work in just enough cold water to bring the pastry together (4–6 tablespoons). When the dough begins to stick together, use your hands to gently knead it into a ball. Wrap the pastry in cling film and put in the fridge to rest while you make the filling.
Heat the oil in a large frying pan over a medium-low heat. Add the onion, carrot and celery and cook gently for 5–10 minutes, or until soft but not coloured.
Add the potato and cook for another five minutes. Stir in the corned beef, making sure it’s evenly distributed.
Add the bottled sauce, pour over the stock and bring to the boil. Lower the heat and simmer, uncovered, for 20 minutes or so, until the carrots and potatoes are tender and most of the liquid has evaporated.
Taste and add salt and pepper if necessary, then tip into a bowl and leave to cool completely.
Preheat the oven to 200C/400F/Gas 6 and have ready a 23cm/9in metal pie plate.
Divide the pastry into two pieces – roughly two-thirds and one third. Lightly flour your work surface and roll out the larger piece to a 2–3mm thickness. Use to line the pie plate. Roll out the remaining pasty ready to form the lid.
Stir the chopped parsley into the filling. Spoon the filling into the pastry-lined dish (if it’s very wet, keep some of the liquid back to serve as gravy). Brush the edges of the pastry on the plate with a little beaten egg, then cover the pie with the pastry lid.
Crimp the edges together with a fork, or your fingers, then trim away the excess pastry neatly. Brush with beaten egg and make a couple of holes in the top to let the steam out. Bake for 30–35 minutes until golden brown. Leave to rest for 15–20 minutes before slicing.