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, 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: dip into the flour now and then to prevent sticking. Using your fingertips, rub the fat into the flour until the mixture resembles fine 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.

Tip: pastry can be chilled for longer if more convenient.

Dice the potatoes into small - medium chunks. Put in a saucepan, more than cover with cold water and season with salt.

Bring to a boil and rapidly simmer until just tender (don't let them get too soft).

Drain and set aside until cold. Cut into smaller chunks if liked.

Skin the onion and chop very finely or grate it.

Tip: if you have a mandoline slicer, it's quickest to finely slice on the mandoline then chop with a knife.

Put the cold cooked potatoes, onions, and grated cheese into a bowl and season with the thyme if using and a generous amount of black pepper. Add a pinch of salt, bearing in mind that Cheddar cheese can be quite salty and you already added salt to the cooking potatoes.

Stir until while combined, taste and add a little more seasoning if liked.

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 you like.

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

Use your hands to press down and firm the filling to reduce the likelihood of gaps.

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 around with the tines of a fork.

Use up any pastry trimmings by re-rolling and cutting out leaves, stars or other shapes. Stick to the top of the pie with some of the 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.

Position a shelf in the middle with a large baking tray on it to preheat: this will help crisp the bottom of the pie and also stop any juices leaking into your oven.

Brush a layer of beaten egg (or milk) all over the pie then make some slits into 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 the filling is hot and just starting to bubble a little through the steam vents.

Tip 1. If your oven cooks unevenly, turn the pie around after the first 20 minutes.

Tip 2. After 20 minutes you can add another layer of beaten egg or milk for a deeper glaze.

When the pie is baked, remove from the oven but leave for 5 minutes before slicing to allow the pastry to firm a little.

Pie can be eaten hot or cold. Store in the fridge for 3 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.