Brown steaks in a little butter. Set aside. Saute onion and mushrooms in dijon mustard. Add wine. Roll out a pastry shell for each steak. Place 1-2 teaspoons mushroom mixture on top of each filet. Wrap pastry around meat, pinching to seal around bottom. Place on a pan with wire rack. Cook 15-20 minutes at 425 degrees. Absolutely fabulous and elegant!