To make the pastry, put the flour, salt and baking powder into a large bowl. Toss through the butter cubes and make a well in the centre. In a jug, whisk together the egg yolk (reserve the white for brushing over the pie before baking) and cream until smooth. Pour the egg mixture into the flour mixture and mix roughly together, leaving the butter as lumps, until you have a sticky dough. Wrap in cling film and chill in the fridge for 20-30 minutes.
Dust the work surface with flour and roll the dough in one direction only, keeping the edges straight and even until you have a rectangular shape that’s 1cm thick – you may need to dust with more flour as the dough is quite sticky. Bits of butter may stick to the worktop, but don’t worry – just scrape them up and put them in the middle of the pastry before you fold it.
Fold the top third down to the centre, then the bottom third up and over that. Give the dough a quarter turn to the right or left and roll out again. Fold as before, then cover with cling film and return to the fridge to chill for another 20-30 minutes.
Repeat this double rolling, folding and chilling two more times – the pastry is now ready to use.
Meanwhile, make the filling. Peel, core and very thickly slice the apples, then toss together with the lemon juice. Put the white sugar in a large frying pan with 1 tablespoon of water. Bring to the boil and cook gently and slowly until it turns to a rich, reddish caramel, then add the sliced apples with lemon and the apple juice. Cook over a high heat, stirring occasionally, until the caramel dissolves, the liquid reduces and the apples start to soften.
Meanwhile, mix the cinnamon and cornflour with 1-2 tablespoons water until smooth, then pour over the apples. Stir well to thicken, then spoon into a bowl and allow to cool.
Butter the inside of a deep 20cm oval pie dish. Roll out two-thirds of the pastry with a little flour until you have an oval-shaped disc that’s large enough to cover the base and overlap the sides of the dish. Line the dish with the pastry, then fill with the apples using a slotted spoon.
Roll the remaining pastry out to make a lid. Trim and crimp the edges together and decorate the top with pastry trimming leaves. Cut slits in the lid to allow steam to escape, brush all over with the beaten egg white and sprinkle over the demerara sugar.
Heat the oven to 180°C, fan 160°C, gas 4. Put the pie on a baking tray (to catch any juices that bubble out) and bake for 50 minutes until golden. Remove from the oven and allow to cool slightly before serving.