For the apple filling:
1) In a small bowl mix together the brown sugar, flour, cinnamon and nutmeg. In another bowl, sprinkle the apple slices with the juice of 1 lemon and toss.
2) Stir the apples into the sugar mixture until evenly coated. Set aside.
For the pastry:
1) In a large mixing bowl, sift together the flour, salt and sugar. Add the fat and break it up with your hands to coat it all with the flour.
2) Add the cold butter cubes and work into the flour with your hands or a pastry cutter. Work quickly, so the butter does not get too soft, until the mixture is crumbly, like fine breadcrumbs.
3) Add the ice water, a little at a time, until the mixture comes together to form a dough. Bring the dough together into a ball; when it sticks together stop working it otherwise the pastry will be over-worked and tough.
4) Divide the pastry in half and flatten each piece slightly to form a disk shape. Wrap each disk in cling film and chill in the refrigerator for about 30 minutes.
5) On a floured surface roll out each disk into a 25 to 28cm circle.
To make the pie:
1) Preheat the oven to 230C/Gas 8.
2) Use 1 round of the pastry to line a 23cm pie dish. Chill the other round.
3) Mound the apple and sugar mixture into the centre of the lined pie dish. Dot with the cubed butter.
4) Using a pastry cutter, cut an even number of strips from the remaining rolled out dough. Transfer alternate strips to the pie top and weave into a lattice by folding every other strip back onto itself and laying another strip at right angles. Fold the strips back into place and repeat until completed.
5) Trim the overhanging dough and crimp the edges. Brush the top of the pie with beaten egg and sprinkle with sugar and cinnamon.
6) Bake the pie for 45 minutes in the oven. Remove and allow it to rest for 20 minutes before slicing.