Prepare the beef tenderloin by trimming away the silverskin and fat, if needed, then tie with butcher's twine at 1-inch intervals. Season the beef generously all over with kosher salt and pepper, planning on about 1 teaspoon of salt per pound of meat.
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.
Next, 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 and then pulse until finely and evenly chopped, about 10-15 pulses.
Repeat this step with the remaining mushrooms, shallots, thyme, and garlic.
After that, 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 to the pan and season with salt and pepper. Stir occasionally, until the mushrooms have released most of their liquid, about 8-10 minutes. Cook for 4-5 more minutes to brown the mushrooms. Remove from the heat and let cool.
On a large piece of plastic wrap, lay out the prosciutto in slightly overlapping layers to form a square large enough to wrap the tenderloin in.
Spread the duxelles (mushroom) mixture evenly over the prosciutto.
Then place the tenderloin on one end.
Roll up the tenderloin, 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 in the fridge for 30 minutes to help it hold together.
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.
Remove the tenderloin from the plastic wrap, then place it 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 to a large baking sheet lined with parchment paper, and then brush with the remaining egg wash. Cut diagonal slashes every inch or so down the length of the pastry to allow steam to vent while it is baking. Sprinkle with flaky salt or coarse sea salt.
Bake until golden brown and the tenderloin reaches 120-125°F on a meat thermometer (for medium-rare), around 40 to 45 minutes, depending on the size of the tenderloin.
Finally, serve slices of beef wellington garnished with minced chives and a sprinkling of flaky or coarse salt.
This 200-year-old recipe is actually fairly easy to put together. The prep time on this easy Beef Wellington recipe is just around 25-30 minutes! Then the rest of the time is spent chilling in the fridge or roasting in the oven!
I know, just 30 minutes of actual work for something that seems so fancy is not unreasonable at all!