PREHEAT oven to 425F.

MELT buter in a skillet and add the onion and mushrooms. Saute until tender and the liquid from the mushrooms are tender. Remove from heat , and add parsley, steak sauce, salt and pepper. Set skillet aside.

BRUSH each steak with one tablespoon of steak sauce, and season with salt and pepper. Cut the pastry sheets in half, placing each steak on the center of a pastry portion. Spread of the mushroom mixture over each steak.

WRAP pastry around the meat. Trim and save any extra pastry. Moisten the ends of the pastry with water and seal closed. Place the Wellington, seam side down, in a shallow; greased baking pan. Brush pastry with beaten egg white. The excess pastry may be re-rolled, cut out and used as decoration for the meat pastries.

BAKE pastries in the oven for 25 minutes or until pastry is golden brown. (Internal temperature for medium-rare is 140F and 155F for medium.)

REMOVE the steak pastries from the oven, and let stand for 5 minutes. The temperature of the steak will rise another 5 degrees during the standing time.