Trim excess fat and silverskin from the tenderloin so it is evenly shaped. Pat the beef dry with paper towels. Season all over with 2 tsp salt and 1 tsp freshly ground black pepper.

Sear

Heat a heavy skillet over high heat until smoking. Add 2 tbs neutral oil. Sear the tenderloin on all sides, including the ends, about 1–2 minutes per side until a deep brown crust forms. Add 2 tbs unsalted butter for the last 30 seconds and baste the beef. Remove the beef to a wire rack to cool until just warm (10–15 minutes).

Mushroom & chestnut duxelles

Prepare mushrooms

Finely chop the mushrooms (or pulse in a food processor in short bursts) until they are small but not pureed.

Cook duxelles

In a large skillet over medium heat, melt 1 tbs butter with 1 tbs olive oil. Add the minced shallots and cook 1–2 minutes until translucent. Add the minced garlic and cook 30 seconds. Add the chopped mushrooms and cook, stirring frequently, until they release liquid and then it evaporates — about 8–10 minutes. Add 2 tbs white wine or sherry and continue cooking until the pan is dry.

Finish duxelles with chestnuts

Stir in the finely chopped chestnuts and 1 tsp chopped fresh thyme. Season with 1 tsp salt and 1/2 tsp black pepper. Cook 1–2 more minutes to incorporate flavors. Remove from heat and cool to room temperature (spread on a plate to cool faster).

Assembly

Prepare prosciutto layer

On a large piece of plastic wrap, arrange 8 slices of prosciutto slightly overlapping in a rectangle large enough to wrap around the tenderloin. Spread the cooled duxelles evenly over the prosciutto, leaving the edges slightly clear so they will seal when wrapped.

Wrap beef in prosciutto

Place the cooled seared tenderloin at one end of the prosciutto/duxelles rectangle. Using the plastic wrap, roll the prosciutto tightly around the beef to form a compact log; twist the plastic wrap ends and refrigerate for 20–30 minutes to set the shape.

Prepare pastry

On a lightly floured work surface, roll the puff pastry into a rectangle about 1/8-inch thick and large enough to fully encase the beef (about 12 x 10 inches). Beat together 2 egg yolks with 1 tsp cold water (and the optional whole egg if you prefer a thinner wash) to make an egg wash.

Wrap in pastry

Remove the plastic-wrapped beef and unwrap. Place the beef in the center of the rolled pastry. Brush the pastry edges with egg wash. Fold the pastry over the beef, trimming any excess so edges meet neatly; press to seal and tuck ends under. Place the seam-side down on a parchment-lined baking sheet. Brush the top with egg wash. If desired, use pastry scraps to decorate the top and brush again.

Chill before baking

Refrigerate the assembled Wellington for at least 20–30 minutes (up to 1 hour) to firm the pastry and help it hold its shape.

Bake

Preheat oven to 425°F (220°C). Make a few small slits in the pastry top to allow steam to escape. Place the chilled Wellington on the center rack and bake for 20 minutes, then reduce oven temperature to 400°F (200°C) and bake an additional 10–15 minutes until the pastry is deep golden and an instant-read thermometer inserted into the center of the beef reads 120–125°F (49–52°C) for medium-rare. Adjust time if you prefer rarer or more done beef.

Rest

Remove from oven and let rest on a cutting board for 10–15 minutes before slicing. Resting brings the internal temperature up a few degrees and allows juices to redistribute.

Optional finish & serving

Slice the Wellington into 1- to 1 1/2-inch thick slices. Sprinkle lightly with coarse sea salt and garnish with chopped parsley. Serve immediately with your choice of pan sauce, red wine reduction, or a simple jus.