Directions

1. Place beef in lightly greased roasting pan. Season with black pepper, if desired. Roast at 425 degrees F for 30 minutes or until meat thermometer reads 130 degrees F. Cover and refrigerate 1 hour.
2. Thaw pastry sheet at room temperature 40 minutes. Preheat oven to 425 degrees F. Mix egg and water.
3. Heat butter in skillet over medium-high heat. Cook mushrooms and onion until vegetables are tender and liquid is evaporated.
4. Unfold pastry on lightly floured surface. Roll into rectangle 4 inches longer and 6 inches wider than beef. Brush with egg mixture. Spoon mushroom mixture onto pastry to within 1 inch of edges. Place beef in center of mushroom mixture. Starting at long sides, fold pastry over beef. Fold in ends. Press edges to seal. Place seam-side down on baking sheet. Brush with egg mixture.
5. Bake for 25 minutes or until golden and thermometer reads 140 degrees F.