Beef Wellington

Prepare the tenderloin.

1. Prepare the beef tenderloin by trimming away the silverskin and fat, if needed, then tie with butcher's twine at 1-inch intervals. Your butcher can do this for you, if you request it. Season generously all over with salt and pepper, planning on about 1 teaspoon of kosher salt per pound of meat.
2. 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. Don't move the meat around in the pan until you are ready to turn it with tongs so it can get a nice sear on all sides, including the ends. This helps seal in the juices.
3. 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.

Make the Duxelles.

1. Combine half of the mushrooms, shallots, thyme leaves, and garlic in a large food processor. Pulse until mushrooms are finely and evenly chopped, about 10-15 pulses. Repeat with the remaining mushrooms, shallots, thyme, and garlic.
2. In the same pan used to sear the tenderloin, heat the butter and olive oil over medium-high heat until melted.
3. 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. Continue to cook the mushroom mixture and 4-5 minutes to brown the mushrooms. Remove from the heat and let cool.

Assemble

1. On a large piece of plastic wrap (you may want to double layer the plastic wrap), lay out the prosciutto in slightly overlapping layers to form a square that will be large enough to wrap the tenderloin in. Spread the duxelles mixture evenly over the prosciutto, then place the tenderloin on one end.
2. Use the plastic wrap to help you roll up the tenderloin in the prosciutto and mushrooms, wrapping it tightly to form a log. Tuck in the ends of the prosciutto as you roll it up. Twist the ends of the plastic wrap closed, then place the tenderloin in the fridge for 30 minutes to help make sure it will all hold together.
3. Preheat oven to 425°F (218°C).
4. Lightly dust a clean surface with flour then roll out the thawed puff pastry, pressing the edges together to seal if working with two sheets. It should create a rectangle large enough to wrap the prosciutto-wrapped tenderloin roast.
5. 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. If you find that you have significant excess pastry along the edges, you may want to trim some of that away.
6. Transfer the wrapped beef wellington to a large baking sheet lined with parchment paper. Brush the top with the remaining egg wash, then cut diagonal slashes every inch or so down the length of the pastry to allow steam to vent while the wellington is baking. Sprinkle with flaky salt or coarse sea salt.
7. Bake until golden brown on the outside and the tenderloin reaches 120-125°F (49-51°C) on a digital meat thermometer for medium-rare doneness. This is typically around 40 to 45 minutes, depending on the size of your tenderloin roast. Remove from the oven and rest for 10 minutes before slicing into thick slices.
8. Serve slices garnished with the fresh minced chives and an additional sprinkling of flaky or coarse salt.