The meat and surrounding layer of chopped mushrooms are wrapped in rice-paper rounds before being swaddled in pastry. This prevents the crust from becoming overly soggy when baked. Centerpiece Roasts
Preheat the oven to 425°. Heat the oil in a large skillet until almost smoking. Add the meat and brown it well on all 4 sides over moderately high heat, about 4 minutes per side. Transfer the meat to a plate and season with salt and pepper. Reserve the skillet for later use.
In another large skillet, melt 3 tablespoons of the butter. Add the mushrooms and cook over high heat, stirring, until lightly browned, about 15 minutes. Season with salt and pepper. Finely chop the mushrooms in a food processor. Add the crème fraîche and process until combined. Season with salt and pepper.
Moisten the rice paper rounds under cool water and lay them on a cutting board until softened. Pat dry with paper towels. On a lightly floured work surface, roll out the puff pastry to a 14-by-12-inch rectangle about 1/8 inch thick. Slightly overlap the rice paper rounds in the center of the pastry. Set the meat on the rice paper rounds and pat the mushroom mixture all over the meat. Wrap the rice papers around the meat to hold the mushrooms in place.
Bring the long sides of the puff pastry up and over the meat, then brush the seam with the egg wash and press to seal. Trim off any excess puff pastry from the short sides and brush with the egg wash. Fold the ends to enclose the meat and set the bundle on a lightly greased baking sheet, seam side down. Brush with egg wash. If desired, cut out leaves or other decorations from the pastry scraps and glue them on with egg wash; brush the decorations with egg wash too.
Bake for 30 minutes, then cover loosely with foil to prevent the pastry from getting too dark. Continue baking until an instant-read thermometer inserted in the center of the meat registers 120° for rare. Transfer the meat to a cutting board and let rest for 5 minutes.
Meanwhile, melt 2 tablespoons of the butter in the skillet used to brown the meat. Add the leek, carrot and shallot and cook over moderately high heat, stirring occasionally, until lightly browned, about 3 minutes. Season with salt and pepper. Add 2/3 cup of the wine and simmer, scraping up the browned juices, until reduced by half, about 5 minutes. Add the remaining 1 cup of wine and the port and simmer over moderate heat until reduced by half again, about 7 minutes.
Cut the remaining 6 tablespoons of butter into small pieces. Pour the sauce through a fine sieve into a small saucepan. Set the saucepan over low heat and gradually whisk in the remaining butter. Season with salt and pepper. Carve the meat into thin slices and serve hot; pass the sauce separately.
Rice paper becomes translucent and pleasantly chewy after being moistened with water. It is available at Asian groceries and some supermarkets.