• Tie butcher's string around the beef tenderloin in 5 or 6 places to keep the cylindrical shape of the beef.
• Season the tenderloin generously with salt and pepper and set aside to come to room temperature while preparing the Duxelles.
• In a large heavy-bottomed skillet set over high heat, add the oil and 1 tablespoon of the butter. When the butter has melted, add the beef tenderloin. Sear the beef 3 minutes on each side and each end to a deep brown.
• Transfer the beef to a plate and remove the string.

For the Duxelles:

• Melt the remaining 2 tablespoons of butter to the skillet and add the mushrooms, shallots and garlic. Cook over medium-high heat, stirring often for 5 minutes. Add the wine, reduce the heat to low and simmer, stirring occasionally, until all the liquid has evaporated and is thick and paste-like, about 20 minutes.
• Remove the mushroom mixture from the heat and stir in the thyme. Add salt and pepper to taste. 
• Transfer the mushroom mixture to a dish and refrigerate to cool.

To Assemble:

• Place a large sheet or overlapping sheets of plastic wrap on your work surface that is about twice as long and wide as the tenderloin.
• Add strips of the prosciutto onto the plastic wrap overlapping them, shingle-style, slightly, to create a square.
• Spread the cooled mushroom mixture evenly on the prosciutto keeping 2-inch border all around.
• Brush the tenderloin on all sides with the Dijon mustard and place on top of the mushroom mixture at the edge closest to you.
• Using the plastic wrap, pull the edge over and begin rolling the prosciutto-mushroom mixture tightly over the beef. Twist the ends of the plastic wrap and place the package in the refrigerator for 15 minutes.
• Preheat the oven to 400°F
• Dust flour on the work surface and lay down a sheet of puff pastry, lightly roll the pastry, both length and width, that will cover the tenderloin.
• Remove the plastic from the beef tenderloin and place on the puff pastry, at the edge closest to you. Brush the far edge with the egg-wash.
• Gently roll the pastry over the beef until it completely encloses the beef roll, seaming it at the bottom as it meets the egg-wash.
• Fold each end of the puff pastry trimming excess with a knife and tucking underneath.
• Brush the pastry covered beef with egg-wash and transfer to the refrigerator for 15-minutes to firm up the pastry.
• Place the Beef Wellington on a parchment-lined baking sheet and brush with egg-wash once more.
• With a sharp knife, lightly cut decorative slits on the surface of the pastry. Sprinkle with Maldon salt flakes or coarse salt.
• Transfer to the preheated oven and bake for 45 minutes or until the pastry is a dark golden color and the internal temperature is 135°F for medium-rare. Cover with aluminum foil if the pastry begins to darken.
• Let rest for 10-minutes before serving. Transfer to a serving platter and garnish with fresh thyme or herbs of choice. Slice 3/4-inch thick.