Roll out the puff pastry on a floured surface to a large rectangle. Spread the mushroom paste in the middle of the pastry (do not cover the sides of the pastry). Remove the cling film from the beef, then lay in the centre of the pastry. Brush the edge of the pastry with egg yolk. Fold the ends over, then wrap the pastry around the beef, cutting off the excess pastry. Turn over, so the seam is underneath, and place on a baking sheet. Brush over all the pastry with egg and chill for about 15 mins to let the pastry rest.