Grease 20cm by 20cm pie tin and preheat the oven to 180°C.
Sift the flour into a bowl, add the salt, add the butter and lard, cut up and rub into the flour until it resembles breadcrumbs. Add very cold water, bit by bit, mixing with a knife, then bring it together with your hands till the pastry forms a ball, try not to add too much water, just enough. Cover in cling film and chill.
Peel and chop the potatoes, put them in a large pan of water, add the vegetable stock cube and boil until tender, for approximately 20 minutes. Drain the potatoes and mash, add the knob of butter and plenty of salt and pepper, and mix it all together.
Saute the onions in the knob of butter until soft and then stir into the mashed potato. Chop the corned beef into small squares and stir it into the potato mix with a fork till all combined well.
Take the pastry from the fridge and let it rest for five minutes before rolling it out. Roll out just over half of the pastry on a floured surface and line the bottom of the tin with it. (Roll out the remaining pastry for the top of the pie and leave to one side).
Fill the pie with the potato and corned beef mixture. Wet the edge of the pie dish, lay on the lid, and with a sharp knife trim the pastry from around the pie. Knock up the sides of the pastry, press around the edge with a fork and crimp the edge of the pie with your fingers.
Make slits in the top of the pie for the steam to escape. Make some pastry leaves with the spare pastry and decorate the pie. Brush the top of the pie with milk.
Place the pie in the middle of the oven for about 40 minutes, but check after 30 minutes.
Tips: Boiling the potatoes with a stock cube in the water adds more flavour. The filling can be made the day before ready to assemble the pie when needed.