Prepare the beef tenderloin by trimming away the silverskin and fat, if needed, then tie with butcher’s twine at 1-inch intervals. Season generously all over with salt and pepper.

Heat a large skillet over high heat until smoking hot, then add the olive oil. Once hot, sear the tenderloin on all sides until well-browned, about 2 minutes per side.

Remove the seared tenderloin from the pan and cut off the twine. Brush it on all sides with the dijon mustard while the meat is still warm. Set aside.

Combine half of the mushrooms, shallots, thyme leaves, and garlic in a large food processor. Pulse until mushrooms are finely and evenly chopped.

In the same pan used to sear the tenderloin, heat the butter and olive oil over medium-high heat until melted. Add the mushroom mixture and season with salt and pepper. Cook, stirring occasionally, until the mushrooms have released most of their liquid, about 8-10 minutes.

On a large piece of plastic wrap, lay out the prosciutto in slightly overlapping layers. Spread the duxelles mixture evenly over the prosciutto, then place the tenderloin on one end. Roll up tightly and refrigerate for 30 minutes.

Preheat oven to 425°F (218°C). Lightly dust a clean surface with flour then roll out the thawed puff pastry.

Remove the tenderloin from the plastic wrap, then place on the puff pastry. Roll it up, tucking the ends under and brushing the beaten egg over the long edge to seal the seam.

Transfer the wrapped beef wellington to a baking sheet lined with parchment paper. Brush the top with the remaining egg wash, then cut diagonal slashes to allow steam to vent.

Bake until golden brown and the tenderloin reaches 120-125°F (49-51°C) for medium-rare doneness, around 40 to 45 minutes. Remove from the oven and rest for 10 minutes before slicing.

Serve slices garnished with fresh minced chives and an additional sprinkling of flaky or coarse salt.