1. In a roasting pan, roast fillet in the middle of a preheated 400°F (200°C) oven for 25 to 30 minutes or until a thermometer registers 120°F (50°C). Let the fillet cool completely and discard larding fat and strings. Skim the fat from the pan juices and reserve juices.
2. In a heavy skillet, cook the mushrooms in the butter over moderately low heat, stirring until all the liquid is evaporated and the mixture is dry; season with salt and pepper and let cool completely.
3. Spread the fillet evenly with the pâté de foie gras, covering the top and sides; spread the mushrooms evenly over the pâté de foie gras. Set aside.
4. On a floured surface, roll 1 pound of the puff pastry into a rectangle large enough to enclose the fillet completely, about 20 by 12 inches.
5. Invert the coated fillet carefully onto middle of the dough and fold up the long sides of the dough to enclose the fillet. Brush the dough's edges with the egg white to seal. Repeat with the ends of dough.
6. Transfer the fillet, seam side down, to a jelly roll pan or a shallow roasting pan and brush with some of the egg wash. Roll out the additional dough and cut shapes with decorative cutters. Arrange the cutouts on top, brush with the remaining egg wash and chill for at least 1 hour, but no more than 2 hours.
7. Bake the fillet in the middle of a preheated 400°F (200°C) oven for 30 minutes, reduce the heat to 350°F (180°C) and bake for 5 to 10 minutes more or until a meat thermometer registers 130°F (55°C) for medium-rare. Remove and let stand for 15 minutes.
8. In saucepan, boil the reserved pan juices and Madeira until the mixture is reduced by one quarter. Add the arrowroot mixture, beef broth, truffles and salt and pepper to taste. Cook over moderate heat, stirring (being careful not to boil) for 5 minutes or until thickened.
9. Loosen the fillet from jelly roll pan, transfer with two spatulas to a heated platter and garnish with watercress. To serve, cut the fillet into 3/4-inch-thick slices, dress with the sauce.
Beef Wellington Recipe © 1991 Epicurious/CondéNet Inc. Photo © 1991 dar1930. All rights reserved. All recipes and photos used with permission of the publisher.