1000 Calorie Beef Wellington

Name: 1000 Calorie Beef Wellington
Ingredients: 1 (3 1/2-pound) beef tenderloin, cleaned and tied with thin strips of bacon, at room temperature; 3/4 pound button mushrooms, chopped fine; 2 1/2 tablespoons unsalted butter; Salt; Pepper; 1/2 cup heavy cream; 1/2 pound pate de foie gras, (available at specialty foods shops) at room temperature; 1 pound frozen puff pastry, thawed, plus additional for garnish, if desired; 1 large egg, separated; 1 teaspoon water; 1/2 cup Sercial Madeira; 2 teaspoons arrowroot dissolved in 1 teaspoon cold water; 1/2 cup beef broth; 2 tablespoons finely chopped black truffles (available at specialty food shops) if desired; Watercress, for garnish if desired