Put the prepared potatoes and onions in a roomy saucepan. More than cover with cold water and season to taste with salt.

Bring to a boil then turn down the heat and simmer until the potatoes are just tender.

Drain* and leave in the colander or sieve to dry while you complete the next step.

*Tip: you're going to make gravy to serve with the pie, save the cooking water and use it as a base for your stock.

Wipe out the saucepan and tip in the potatoes and onion along with the chopped corned beef and ¼ tsp pepper.

Use a potato masher to break down and combine the ingredients: leave it as chunky or mash as smooth as you like.

Taste and add more salt or pepper if needed.

Transfer to a suitable container, set aside until cool, then put in the fridge until completely cold.

Using a food processor

Put the flour and salt into the machine and briefly process to combine.

Dice the butter plus lard if using, add to the machine and process until the mixture looks like fine breadcrumbs.

With the motor running add cold water, 15ml / 1 tbsp at a time, until the mixture comes together into a smooth ball.

Making by hand

In a medium sized bowl, stir together the flour and salt.

Coarsely grate in the butter plus lard if using, dipping into the flour now and then to prevent sticking. Using your fingertips, rub the fat into the flour until the mixture resembles breadcrumbs.

Add 2 tbsp of cold water and stir in with a round bladed knife. Continue adding tablespoons of water and stirring until the mixture starts coming together.

Use your hands to bring together into a smooth ball.

Using digital scales, divide the pastry in two: roughly 60 per cent for the pie base and 40 percent for the lid. e.g. if the dough weighs 420g divide into 250g for the base and 170g for the lid.

Flatten each piece into a disc then wrap in foil or cling film and chill in the fridge for at least 30 minutes: this makes it easier to roll out.

On a lightly floured work surface, ideally covered with a silicone mat to reduce sticking, roll out the larger piece of pastry into a circle that will come over the edge of your pie plate: remember to take account of any dip in the middle.

Place the pastry on the pie plate and gently press down. Holding up the plate in one hand, go around the edge with a knife to trim away excess pastry.

Roll out the smaller piece of pastry a little bigger than your pie plate and set aside.

Tip: the cut-out pastry can be chilled again if convenient.

Transfer the filling to the pastry-lined plate and spread it evenly, leaving a bare border approximately 2 cm all round.

Brush some of the beaten egg (or milk) all over the border. Place the pastry lid on top, pressing down to expel any air trapped underneath, then firmly press all around to stick it to the base. Use a knife to cut off the excess pastry.

Go all around pinching the edge between two fingers to create a wavy pattern or press with the tines of a fork to make lines.

Use up any pastry trimmings by re-rolling and cutting out shapes to decorate the pie. Stick to the top with beaten egg (or milk).

Tip: if you don't want to cook the pie straight away, it can be chilled in the fridge.

Preheat your oven to 200°C / 180°Fan /Gas 6 /400°F with a shelf in the middle position.

Tip: preheat a large baking tray on the shelf which will help crisp the bottom of the pie.

Brush a layer of beaten egg (or milk) all over the pie then make 4 short slits in the pastry to allow steam to escape.

Transfer to the preheated tray in the oven and bake for 35 minutes or until the pastry is crisp, golden brown and there are juices bubbling from the steam vents.

Tips: After 20 minutes add another layer of beaten egg or milk for a deeper glaze. If your oven cooks unevenly, turn the pie around too.

When the pie is done, remove from the oven but leave for 5 minutes so the pastry can firm a little before cutting into it.

Can be eaten hot or cold. Store in the fridge for 2 days or freeze.

Reheat in a low oven, air fryer, or microwave. Recommended: reheat in a microwave followed by a few minutes in an air fryer or medium oven to re-crisp the pastry.