Make the rub for the tenderloin, adding just enough olive oil to a make it a paste.
Truss the tenderloin so that it's as round as possible. Smear the rub all over the tenderloin.
Drizzle a bit of oil in a pan and once hot, sear the loin all around, including the ends. Set aside to cool. Keep those pan drippings and set the pan aside for the green peppercorn sauce!
Once cooled down, remove the ties and smear all over with the duxelles. Roll tightly in saran wrap and set in fridge to chill.
Roll out the puff pastry in a large rectangle. Wrap the tenderloin, sealing with a bit of egg wash, making sure to seal the ends well.
Place on a foil lined cookie sheet, seam side down, brush with egg wash.
Set your thermometer for 135F and bake in a preheated oven at 425F for approximately 45 minutes to an hour.