Let beef come to room temp and wrap with twine to hold its shape while cooking.
Season beef with a pinch of salt and pepper on all sides.
Add a tablespoon of olive oil to a hot pan, (preferably cast iron) then add the beef and sear all sides. This should take about 1 minute. Remove from heat and place on a plate or cutting board and remove twine.
While the beef is still hot, brush generously with English Mustard. TIP: this cut of meat has very little fat, so it is extremely tender but slightly less flavorful because of this. Do not be afraid to go overboard with the english mustard as it will not overpower the beef at all and will provide a delicious flavor as it is cooked into the meat. We used every bit of ¼ cup.
Add olive oil, sea salt, thyme, mushrooms, pepper, and garlic to a food processor and pulse until pureed.
Add mushroom mixture to a hot pan on stovetop over medium high heat. Saute for 5 minutes to remove moisture from the mushrooms. (The point of this is to cook most/all of the moisture out of the mushrooms- as they cook they release moisture which would make the puff pastry soggy if not done beforehand)
Lay out a sheet of plastic wrap. In the center, lay out prosciutto the approximate length of your full length of the beef tenderloin, overlapping each piece. (The amount of prosciutto will vary based on the size/cut, for us it took 7 pieces)
Spoon mushroom duxelles onto prosciutto and spread to evenly cover. Place beef on top in the center and tightly roll the prosciutto over the beef using the plastic wrap. Tuck prosciutto under as you roll. Twist the ends of the plastic wrap tightly, this will help hold everything together tight. Chill in the fridge for 30 minutes.
Preheat oven to 425*
Roll out puff pastry sheets side by side on a lightly floured surface. Pinch the seams to connect the two sheets. Place pastry atop a new sheet of plastic wrap. (as for the size of the plastic wrap, you will want about 6” past the length of the puff pastry on the ends you will be rolling)
Remove beef from the fridge and remove plastic wrap. Place into the center of the puff pastry. Generously brush all sides with egg wash. Roll puff pastry tightly over the beef and trim excess pieces. Fold the sides and seal with egg wash. Tightly wrap in plastic wrap, twisting the edges to tighten the wrap and place in the fridge for 5 minutes.
Remove from the fridge and discard plastic wrap. Brush the entire pastry with egg wash. Use a knife to score the top of the pastry and sprinkle with salt and pepper.
Place into the center rack of the oven. Bake in preheated oven at 425 degrees F for 45 minutes. Let rest 10 minutes after cooking before serving. Garnish with fresh parsley.