To make the Duxelles add your shrooms, shallots, garlic, and herbs to a food processor and pulse until finely chopped.
Add butter and olive oil to a large pan over medium heat. Add the mushroom mixture to the pan and let it heat until the moisture has evaporated.
Season with salt and pepper and chill in the refrigerator for a while moving to the next steps (this will help the wrapping process).
To prepare the beef:
Tie the tenderloin. Its a big daddy so I recommend doing it in 6 places so it holds its cylindrical shape while cooking.
Rub your meat with olive oil, then season it all over with salt and pepper. Sear it all over, including the ends, in a hot skillet or flat grill.
Now for the prosciutto!
Roll out a sheet of plastic wrap on top of your cutting board. For some meat this size… we did several sheets overlapping on the counter about 3 feet by 3 feet.
Shingle the prosciutto so it forms a rectangle that is big enough to encompass the entire filet of beef.
Using a rubber spatula, now cover it all evenly with a layer of your duxelles. You can sprinkle salt, pepper and fresh thyme leaves all over this.
When the beef is seared, remove from heat, cut off twine and smear lightly all over with Dijon mustard. Then roll up your meat in the duxelles covered prosciutto using the plastic wrap to tie it up nice and tight.
Tuck in the ends of the prosciutto as you roll to completely encompass the beef. Roll it up tightly in plastic wrap and twist the ends to seal it completely and hold it in a nice log shape.
Let it cool for 30 minutes to ensure it maintains its shape…
Preheat oven to 425 degrees F.
On a lightly floured surface, roll the puff pastry out. We overlapped these sheets and pressed them together, as our meat is huge.
Remove beef from the “refrigerator” and cut off the plastic. Set the beef in the center of the pastry and fold over the longer sides, brushing with egg wash to seal. Trim ends if necessary then brush with egg wash and fold over to completely seal the beef.
Brush the top of the pastry with egg wash then make a couple of slits in the top of the pastry using the tip of a paring knife - this creates vents that will allow the steam to escape when cooking.
Bake for 60 to 70 minutes until pastry is golden brown and beef registers 125 degrees F on an instant-read thermometer. Remove from oven and rest it for a little while before cutting into thick slices.
Green Peppercorn Sauce:
Ensure you’re in a fire safe area… Add olive oil and beef dripping to a sauté pan. Add shallots, garlic, and thyme; sauté for 1 to 2 minutes hot. Then, off the heat and add brandy. Flambé it using a long kitchen match or lighter.
After the flame dies down, return to the heat, add stock and reduce by about half.
Strain out solids, then add 2 cups cream. Reduce by half again, then shut off heat and add green peppercorns.
Garnish your Wellington with minced chives or onions, and serve with Green Peppercorn Sauce. It should be a beautiful medium rare.