Finely mince the mushrooms and garlic, then sauté in olive oil over medium heat until all moisture has evaporated to create a dry duxelles.
Season the beef tenderloin with sea salt and black pepper on all sides.
Sear the beef in a hot pan for approximately 1 minute per side until a brown crust forms, then remove from heat.
Brush the warm beef immediately with Dijon mustard and let it rest.
Lay a piece of plastic wrap on a flat surface, place the prosciutto slice down, and spread the mushroom duxelles evenly over the prosciutto.
Place the beef in the center and roll it tightly within the prosciutto and plastic wrap, chilling in the refrigerator for 15 minutes to firm up.
Preheat your oven to 425°F (220°C).
Roll out the puff pastry, unwrap the chilled beef, and place it in the center of the pastry sheet.
Fold the pastry over the beef to seal it completely, trimming any excess, and brush the exterior with a beaten egg wash.
Bake for 15 to 20 minutes until the pastry is deeply golden and the beef reaches an internal temperature of 130°F for medium-rare.