In a roasting pan, roast the beef in the middle of a preheated 400f oven for 25-30 minutes or until a meat thermometer registers 120f. Let the fillet cool completely and discard the larding fat and strings. Skim the fat from the pan juices and reserve the juices. In a heavy skillet, cook the mushrooms in butter over mod-low heat, stirring, until all liquid they give off is evaporated and the mixture is dry. Season with salt and pepper and let cool completely. Spread the fillet evenly with foie gras, covering the top and sides, and spread the mushrooms evenly over the foie gras. On a floured surface, roll one lb. of puff pastry into a rectangle about 20x12 inches, or large enough to enclose the roast completely. Invert the fillet in the middle of the pastry and fold up the long sides to enclose it, brushing the edges of the dough with some egg white to seal them. Fold the ends of the dough over the fillet and seal with the egg white. Transfer the fillet, seam side down, to a jelly-roll pan and brush the dough with egg wash. Roll out additional dough and cut out shapes with which to decorate the dough. After applying, brush with egg wash. Chill for at least 1 hour and up to 2 hours. Bake the fillet in the middle of a preheated 400f oven for 30 minutes. Reduce heat to 350f and bake an additional 5-10 minutes more or until a meat thermometer registers 130f for medium rare meat and the pastry is cooked through. Let stand for 15 minutes. In a saucepan, boil the reserved pan juices and the Madeira until the mixture is reduced by one fourth. Add the arrowroot mixture, broth, truffles, and salt and pepper to taste. Cook the sauce over moderate heat, stirring, for 5 minutes or until thickened, being careful not to let it boil. Loosen the fillet from the pan, transfering it to a heated platter. Garnish with watercress. Serve the fillet cut into 3/4-inch thick slices, with the sauce. a 1966 Gourmet Mag. favorite.