Directions

1. Preheat broiler. Melt butter in large skillet. Add mushrooms and onion. Cook, stirring occasionally, over medium heat for 15 minutes or until mushrooms are tender and liquid is absorbed. Remove from heat; cool slightly. Season with salt and pepper.
2. Place pork tenderloin on rack in broiler pan. Broil 4 inches from heat for 8 to 10 minutes, turning over halfway during broiling. Transfer tenderloin to plate. Move oven rack to center of oven; heat oven to 400 degrees F.
3. Place large sheet of waxed paper on counter. Roll pastry to a rectangle about 2 inches longer and 8 inches wider than tenderloin. Spread on mushroom mixture to within 2 inches of edges. Pat tenderloin dry with paper towels; place tenderloin on top of mushroom mixture in center. Use waxed paper to help lift long sides of pastry up and tightly around tenderloin, overlapping center edges; pinch edges together to seal. Fold short ends of pastry up; pinch to seal. Place pastry-wrapped tenderloin, seam side down, on parchment-lined baking sheet.
4. Bake for 25 to 35 minutes or until internal temperature is 145 degrees F. and pastry is golden. Transfer to cutting board. Loosely cover with foil; let rest 5 to 10 minutes.
5. Prepare sauce mix according to package directions, if desired. Cut pork into 1/2 -inch-thick slices; arrange on serving platter. Serve with sauce.