Roll out half the pastry and use it to line a 23cm / 9 in pie plate.
Mix the chopped sage into the sausage meat, season well with sea salt, black pepper and the ground nutmeg.
Place half of the sausage meat over the pastry base, leaving a border around the edge. Cover with the thinly sliced apples, then top with the rest of the sausage meat mixture.
Roll out the rest of the pastry. Brush the border of the bottom half of the pie with the beaten egg and cover with the remaining pastry, pressing down well to seal. Crimp the edges of the pie and cut a cross in the centre for the steam to escape whilst cooking.
Place in the fridge for 30 minutes.
Brush the pie with the remaining beaten egg and cook in the preheated oven for 10 minutes, before turning the heat down to 180°C/350°F/Gas Mark 4 and cooking for a further 30 – 40 minutes, until puffed up and golden brown. Leave for 10 minutes before serving.