If making your own puff pastry, prepare it up to the final stages and leave to chill.

Peel and slice the potatoes to a thickness of 3/4cm and boil in a pan of water for 3-5 minutes until just tender, but still firm. Drain the potatoes and leave to cool.

While the potatoes are cooking, thinly slice the onion and heat a tablespoon of oil in a frying pan. Fry the onions for 10 minutes until they are soft and translucent, don't let the onions colour at all, add a touch of water if you need to. Leave to one side to cool.

Grate the cheddar and place into a bowl that is big enough to fit the onions as well. Add the cooled onions, the cream and the salt and pepper. Stir to combine and taste a small amount of the mixture, add more salt and pepper if necessary.

Preheat the oven to 200C/180C Fan and line a baking tray with a silicone mat or a piece of greaseproof paper.

Take the puff pastry and divide into two, one batch of 200g and one of 300g. Roll the 200g piece of pastry out and cut out a circle that is 23cm diameter and place it on the prepared baking tray.

Leaving a gap of a couple of centimetres around the edge of the pastry place a layer of potatoes and add a layer of the cheese and onion mix, keep layer to form a mound and finish with a layer of cheese and onion mix.

Brush the edge with egg and roll out the other block of pastry to make a 24cm circle, place this over the top, match up the edges and press down the pastry to secure it. Cut a circle out of the top of the pithivier to let the air out. You may need to trim the edges to neaten it.

Brush the pithivier with egg wash and then place it in the fridge to chill for 15 minutes.

Take the chilled pithivier from the fridge and using a sharp pointed knife lightly score the pastry with curved lines spiralling out from the top.

Place the pithivier in the preheated oven and cook for 30 - 35 mins until the pithivier is golden. You can serve the pithivier warm or leave it to cool and eat it cold with salad.