1. Take out a package of Pepperidge Farm® Puff Pastry Sheets and remove one from the box. Let it thaw on the counter for 40 minutes - or overnight in the fridge. Return the second to the freezer.
2. Preheat the oven to 425°F.
3. Place the beef tenderloin into a lightly greased roasting pan. Season it with black pepper, if desired. When the oven reaches temperature, roast the tenderloin for 30 minutes or until a meat thermometer reads 130°F.
4. Cover the pan and refrigerate the meat for an hour to cool.
5. Reheat the oven to 425°F.
6. Make an egg wash by beating an egg with a tablespoon of water in a small bowl using a fork or whisk.
7. Heat one tablespoon of butter in a medium skillet over medium-high heat. Add the mushrooms and onion and cook until the mushrooms are tender and all the liquid is evaporated, stirring often. Let the mushrooms cool.
8. Lightly flour your work surface. Unfold one pasty sheet and roll it into a rectangle about 4 inches longer and 6 inches wider than the beef. Brush the pastry sheet with the egg mixture.
9. Spoon the mushroom mixture onto the pastry sheet to within one inch of the edges. Place the beef in the center of the mushroom mixture, fold the pastry over the beef and press to seal.
10. Place the Wellington, seam-side down onto a baking sheet. Tuck the ends under to seal. Brush the pastry exterior with the egg wash.
11. Bake the Wellington for 25 minutes or until the pastry is golden brown and a meat thermometer reads 140°F.