Directions

1. 1

   Season the beef tenderloin generously on all sides with kosher salt and freshly ground black pepper. Heat 2 tablespoons of olive oil in a large skillet over high heat until shimmering. Sear the beef on all sides for 8 to 10 minutes until deeply browned. Transfer to a baking sheet and allow to cool completely at room temperature for about 20 minutes.

2. 2

   Cut the mushrooms into chunks. Add half of the mushroom pieces to a food processor and pulse until very finely minced. Transfer to a bowl and repeat with the remaining mushroom pieces. Set aside.

3. 3

   In a large pan over medium-high heat, melt the butter with the remaining 1 tablespoon of olive oil until foaming. Add the minced shallot and cook for about 4 minutes, stirring occasionally, until completely softened. Add all the chopped mushrooms and sauté for approximately 10 minutes, stirring frequently, until all the liquid they release has evaporated and the mixture appears dry. Pour in the heavy cream and simmer for about 2 minutes until thickened. Remove from heat and fold in the breadcrumbs and fresh parsley. Transfer the mushroom mixture to a small bowl, let cool to room temperature, then cover and refrigerate for at least 1 hour until it resembles a thick paste.

4. 4

   In a small bowl, whisk together the Dijon mustard and prepared horseradish until combined. Once the cooled beef is cool to the touch, carefully remove and discard the butcher's twine. Spread the mustard-horseradish mixture evenly over all sides of the beef. Place on a plate, cover loosely, and refrigerate for 1 hour.

5. 5

   Preheat the oven to 400 degrees Fahrenheit. Position a rack in the center. Line a baking sheet with parchment paper.

6. 6

   Crack the egg into a small bowl and whisk until smooth. Lightly dust a work surface with flour. Unroll the thawed puff pastry sheet and use a rolling pin to extend it so it measures approximately 4 inches longer than the beef tenderloin and 6 to 7 inches wider. Brush a 3-inch border along one long edge of the pastry with the beaten egg. Spread and gently pat the mushroom duxelles over the top and all sides of the beef. Place the beef mushroom-side down in the center of the pastry sheet. Pat any remaining mushroom mixture onto the exposed top side of the beef. Fold the two short ends of the pastry up and over the beef ends, tucking them in. Fold the long sides up and over, wrapping the beef completely. Place the wrapped beef seam-side down on the prepared baking sheet. Brush the entire pastry surface with egg wash. Using a small sharp knife, cut 3 decorative slits into the top and lightly sprinkle with flaky sea salt.

7. 7

   Roast in the preheated oven for approximately 40 minutes, until an instant-read meat thermometer inserted into the thickest part of the beef (avoiding the pastry and mushroom layer) reads 120 degrees Fahrenheit. Remove from the oven and let rest undisturbed for 20 minutes; the internal temperature will continue to rise to approximately 125 degrees Fahrenheit, resulting in a beautiful medium-rare with a ruby-red center. Using a sharp knife, slice into 1-inch-thick medallions and serve immediately.