Allow patty shells to thaw 2 hours in refrigerator before rolling out. If meat is quite long (Lin, control Thyself!!), fold narrow end under to form a roast about dozen to 33 cm in length. Place on rack in shallow roasting pan. Sprinkle lightly with salt and flour. Bake, uncovered, in 220ºC oven until meat thermometer inserted in thickest portion registers 50ºC , about 35 to 40 minutes. Cool meat sufficiently to handle, about 20 minutes. Meanwhile, in skillet, cook mushrooms in butter until tender, about 5 minutes. Remove from heat. Add pate, Cognac and chives. Make a deep slit in center of roast, cutting from side, to but not through opposite side. Sprinkle pocket with salt and pepper. Spoon mushroom-pate mixture into pocket. On lightly floured surface, arrange patty shells in a rectangle, overlapping edges slightly; press edges to seal. (If necessary, moisten edges with a little water.) Roll out to a 16x11-inch rectangle. Place roast in a greased 15x10x1-inch baking pan. Lay pastry over roast, tucking under securely. Brush pastry with mixture of egg yolk and water. Bake in 220ºC oven for 20 minutes more, or until pastry is golden. Makes 8 servings.