Preheat oven to 425. Bake filet for 20 minutes. Prepare mushroom sauce: Chop mushrooms and onions very finely and saute in oil and butter until golden. Season with nutmeg, salt, and pepper. Douse filet with brandy warmed to fragrance point and flambe. Let meat and sauce cool. Roll out puff pastry until large enough to go around filet. Spread mushroom sauce on pastry, add filet, close and seal pastry with egg white and brush it over outside of crust. Bake for 10 minutes. Reduce heat to 375 for 20 minutes, until crust is a delicate golden brown colour. Let the bundle stand for 15 minutes before carving into 3/4" slices.
