Preheat a large cast-iron skillet over high heat until smoking. Season the beef tenderloin generously with 1/2 tsp salt and 1/4 tsp black pepper.

Add 2 tbsp olive oil to the hot skillet. Sear the beef tenderloin on all sides until deeply browned, about 2-3 minutes per side. This should take about 8-12 minutes total.

Remove the beef from the skillet and let it cool slightly on a wire rack. Once cool enough to handle, brush all sides of the beef with the Dijon mustard. Set aside.

In the same skillet (or a clean one), heat 1 tbsp olive oil and 1 tbsp butter over medium-high heat.

Add the finely chopped mushrooms, minced shallots, and minced garlic. Cook, stirring frequently, until all the liquid has evaporated and the mushrooms are deeply browned and caramelized, about 15-20 minutes. A food processor can be used to finely chop the mushrooms.

If using, add the dry sherry or red wine and cook until fully evaporated. Stir in the chopped fresh thyme and parsley. Season with 1/2 tsp salt and 1/4 tsp black pepper.

Spread the duxelles onto a plate or shallow dish to cool completely. This step is crucial to prevent the pastry from becoming soggy.

Lay a large sheet of plastic wrap on a clean work surface. Arrange the prosciutto slices slightly overlapping to form a rectangle large enough to cover the beef tenderloin.

Evenly spread the cooled duxelles over the prosciutto, leaving a small border around the edges.

Place the seared and mustard-coated beef tenderloin in the center of the duxelles-covered prosciutto.

Using the plastic wrap, carefully lift the prosciutto and duxelles to wrap it tightly around the beef. Roll it up firmly, twisting the ends of the plastic wrap to create a tight, uniform log. Refrigerate for at least 30 minutes (or up to 24 hours) to allow it to set.

On a lightly floured surface, roll out the thawed puff pastry sheet into a rectangle large enough to completely encase the beef log, about 12x18 inches. You may need to trim the edges to make them straight.

Unwrap the chilled beef log and place it in the center of the rolled-out puff pastry.

Brush the edges of the pastry with some of the beaten egg wash. Carefully fold one side of the pastry over the beef, then fold the other side to overlap, sealing the seam. Trim any excess pastry, leaving about a 1-inch overlap. Press firmly to seal.

Fold in the ends of the pastry and seal them securely, trimming any excess. Ensure the Wellington is completely sealed.

Carefully transfer the Beef Wellington, seam-side down, to a baking sheet lined with parchment paper.

Brush the entire surface of the pastry with the remaining egg wash. If desired, score the top of the pastry lightly with the back of a knife in a decorative pattern (avoid cutting all the way through).

Refrigerate the assembled Wellington for another 15-30 minutes. This helps the pastry hold its shape and prevents shrinking.

Preheat oven to 400°F (200°C).

Bake the Beef Wellington for 35-45 minutes, or until the pastry is golden brown and flaky, and an instant-read meat thermometer inserted into the center of the beef registers 125-130°F (52-54°C) for medium-rare. For medium, aim for 130-135°F (54-57°C).

Remove from the oven and let rest on a cutting board for 10-15 minutes before slicing into thick pieces. Resting is crucial for the juices to redistribute.