Heat the oil in a pan and gently fry the onion until soft. Add the mushrooms and garlic and continue to cook for a further 2-3 minutes.
Add the sage and cider and bring to the boil then simmer for 5-10 minutes until the liquid is reduced by half the quantity. Stir in the Quorn pieces, chestnuts and cream. Add the corn flour stirring until the sauce has thickened. Remove from the heat, cool and refrigerate for 30 minutes.
Preheat the oven to 200C/400F/Gas Mark 6.
Roll out the pastry to a thickness of approximately half a centimetre into a rectangular shape and spoon the Quorn Pieces mixture into the centre. Fold the pastry over the mix to form a tube and tuck the pastry edge firmly underneath then fold in the ends to seal the tube. Brush generously with beaten egg then place on a greased baking tray and bake in the centre of the oven for 30-35 minutes until golden brown.