Split the pastry into a third and two thirds, and roll out the larger piece on a floured surface into a circle about 36cm round – large enough to line a 22cm springform tin that’s 6cm deep and leave an overhang all around.
Press the pastry into the tin and up the sides. Roll out the other piece of pastry into a circle 25cm in diameter and place on a baking sheet. Put this and the lined tin in the fridge.
Preheat the oven to 200C/190C fan*/gas mark 6 and put an empty baking sheet in to heat up. Peel the potatoes and cut them into fine slices – a mandolin is the best thing for this. As you slice them, drop them into a large bowl of cold water.
Heat the butter and oil in a heavy-bottomed pan and cook the onion for 7 minutes, stirring occasionally. Add salt and pepper, the garlic and a splash of water. Turn the heat down low, cover the pan and sweat the onions for 15 minutes.
Keep an eye on them – you want them to be soft but not caramelised. Stir in the parsley. Scrape into a dish and leave to cool completely. Drain the potatoes and dry them well using kitchen paper or clean tea towels.
Layer the components of the filling in the pie, starting with a layer of potatoes, then onions, then spring onions, then cheese. Season the potatoes as you do this, and add some nutmeg. You should be able to make two layers of each component, then finish with a final layer of potatoes.
Wet the bottom edge of the circle of pastry with cold water. Place it on top of the pie, pressing down on the overhang to seal it. Cut away the excess dough with scissors, leaving a 1-2cm overhang all the way round.
Roll the overhang up on to the top of the tart so it forms a rim. Decorate the top with the trimmings.Make a vent in the top by cutting a large X in the centre of the pie and pulling back each pastry point. Brush the tart with the egg wash.
Slide the tin on to the heated sheet and bake for 30 minutes, then lower the temperature to 180C/170C fan*/gas mark 4 and cook for another 30 minutes. If the top is getting too dark, cover the pie with a foil tent.
Check for tenderness by putting the point of a sharp knife right into the pie. The potatoes should be quite soft.
Heat the cream until it’s hand-hot. Season. Bring the pie out of the oven and slowly pour the cream in through the vent in the top, allowing it to sink all the way through the potatoes.
If it doesn’t fit, make a couple of neat slits through the lid and into the potatoes, and pour the rest into these, tipping the pie to make sure the cream is spreading.
Put the pie back in the oven and bake for another 30-40 minutes. Leave to cool for 15 minutes. This allows the potatoes to ‘set’ a little. Run a knife between the tin and the edges of the pie.
Unclasp the tin at the side and carefully remove the ring. Run a palette knife under the pie and slide it on to a warm serving plate.