The crust seals in the juices in the turkey.
1. Heat the oil and butter in a skillet and cook the turkey over a medium to high heat for 2-3 minutes on each side, until sealed and beginning to brown. Leave to cool.
2. Fry the celery lightly for 1-2 minutes and then mix with the walnuts and cheese.
3. Divide the pastry into 4 and roll out each piece on a floured surface to a square large enough to encase each turkey breast.
4. Place a turkey breast onto a square of the pastry, top with the celery, nut, and cheese mixture. Wrap the pastry around to form a parcel, sealing the edges well with a little water. Brush with beaten egg and place onto a baking sheet.
5. Bake in a preheated oven 425°F, for 20-25 minutes until risen and golden brown.