Preheat the oven to 200ºC/400ºF/Gas 6.

Roll out the pastry and cut 4 rectangular pieces large enough to enclose the steaks.

Gently fry the mushrooms and garlic in the butter, adding sufficient breadcrumbs to form a malleable stuffing.  Rub a heavy pan with lard and sear the steaks on both sides.  Place an eighth of the stuffing in the centre of a piece of pastry, place a steak and top of the mixture with another eighth of the stuffing on top of the steak.

Repeat with the other steaks.

Brush the pastry edges with the beaten egg yolk and enclose the steak, sealing the edges.  Brush the parcels all over with beaten egg and bake for 20 minutes or until golden.