Preheat and prepare: Preheat oven to 400°F (200°C). Season beef tenderloin generously with kosher salt and black pepper on all sides.

Sear the beef: Heat olive oil in a large skillet over high heat. Sear beef on all sides until golden brown, about 2-3 minutes per side. Don't move the beef until it has had time to brown properly. Remove from heat and let cool completely.

Apply mustard: Once cooled, brush the seared beef on all sides with Dijon mustard. Set aside.

Make mushroom duxelles: Pulse mushrooms in food processor until finely chopped. Cook in a dry skillet over medium-high heat, stirring frequently, until all moisture evaporates (10-15 minutes). The mixture should be dry. Cool completely.

Wrap in ham and mushrooms: Lay plastic wrap on work surface. Arrange prosciutto slices in overlapping pattern large enough to wrap around beef. Spread cooled mushroom mixture over ham. Place beef in center and wrap tightly using plastic wrap. Refrigerate for 20 minutes.

Prepare pastry: Roll puff pastry on lightly floured surface to size that will wrap around beef. Unwrap beef from plastic and place in center of pastry.

Wrap in pastry: Brush pastry edges with beaten egg yolk. Wrap pastry around beef, sealing seams and trimming excess (avoid thick overlaps). Place seam-side down on baking sheet.

Final touches: Brush entire surface with egg wash. Score decoratively with sharp knife (don't cut all the way through). Sprinkle with flaky salt. Chill for 5-10 minutes.

Bake: Bake for 25-35 minutes until pastry is golden brown. Check internal temperature with instant-read thermometer - aim for 125-130°F for medium-rare.

Rest and serve: Remove from oven and let rest for 10 minutes before slicing. Cut into 1-inch thick slices with sharp knife. Serve immediately.