Season the beef tenderloin generously with salt and pepper.

Heat olive oil in a large skillet over high heat.

Sear the beef on all sides until browned, about 2 minutes per side.

Remove from heat, brush with Dijon mustard, and let cool completely.

Melt butter in a skillet over medium heat.

Add mushrooms (and shallot if using).

Cook until all moisture evaporates and the mixture is dry.

Season lightly with salt and thyme.

Cool completely.

Lay plastic wrap on a work surface.

Overlap prosciutto slices into a rectangle.

Spread mushroom duxelles evenly on top.

Place the beef in the center and roll tightly.

Wrap firmly and refrigerate for 30 minutes.

Roll out puff pastry slightly on a floured surface.

Remove plastic wrap from beef and place on pastry.

Wrap pastry tightly around beef and seal edges.

Brush with egg wash and chill for another 30 minutes.

Preheat oven to 400°F (200°C).

Place Wellington seam-side down on a lined baking sheet.

Brush again with egg wash.

Bake for 40–45 minutes, until pastry is golden and internal temp reaches 125–130°F for medium-rare.

Rest for 10 minutes before slicing.