Pre-heat the oven to 200°C/180°C (fan)/gas mark 6. Cook the apples in a lightly greased baking dish until soft. Remove from the oven, purée and allow to cool.
Pulse the biscuits in a food processor or crush in a polythene bag with a rolling pin until broken up into small pieces. Mix with the apples and remaining ingredients (apart from the extra beaten egg) and beat well until blended.
Roll out the puff pastry and line a greased round pie dish with it. Prick the pastry several times with a fork. Remove the excess pastry and slice into long thin strips, of about 1cm width. Fill the pie with the apple mixture, cover with the lattice strips and brush the pastry edges with the beaten egg.
Bake in the pre-heated oven for 25-30 minutes until the pastry is golden brown. Serve warm or cold. Delicious with custard.