Pat the 2 lb center-cut beef tenderloin trimmed and tied if uneven completely dry with paper towels — this is essential for a good sear. Season generously on all sides with 1 tsp kosher salt and 1 tsp kosher salt.

Heat 2 tbsp olive oil in a Cast Iron Skillet over high heat until smoking. Sear the 2 lb center-cut beef tenderloin trimmed and tied if uneven on all sides, including the ends, until deeply browned — about 2 minutes per side (6–8 minutes total). You are building crust and flavor, not cooking through. Transfer to a Wire Rack and let cool completely.

Once the 2 lb center-cut beef tenderloin trimmed and tied if uneven is cool to the touch, brush a thin, even layer of 2 tsp Dijon mustard over the entire surface. Refrigerate uncovered while you prepare the duxelles.

Pulse the 1 lb cremini mushrooms roughly chopped in a Food Processor until very finely chopped — nearly a paste. Work in batches if needed. Do not add liquid.

Heat 2 tbsp unsalted butter and 2 tbsp olive oil in the same Cast Iron Skillet over medium-high heat. Add the 1 lb cremini mushrooms roughly chopped, 4 cloves garlic minced, and 2 tsp fresh thyme leaves. Cook, stirring frequently, until all the moisture has evaporated and the mixture is dark, dry, and beginning to stick slightly to the pan — about 12–15 minutes. Add the 2 tbsp dry sherry (if using) and cook 1 more minute. Season with 1 tsp kosher salt and 1 tsp kosher salt.

Spread the duxelles onto a plate in a thin layer and refrigerate until completely cool, at least 20 minutes. The duxelles must be cold and dry before assembly — any warmth or moisture will make the pastry soggy.

Lay a large sheet of Plastic Wrap on your work surface. Arrange the 12 slices prosciutto di Parma thin-sliced slices in a rectangle, overlapping slightly, large enough to wrap the entire 2 lb center-cut beef tenderloin trimmed and tied if uneven (roughly 10x12 inches). Spread the cooled duxelles evenly over the 12 slices prosciutto di Parma thin-sliced in a thin, even layer, leaving a 1-inch border on all sides.

Place the mustard-coated 2 lb center-cut beef tenderloin trimmed and tied if uneven at the bottom edge of the 12 slices prosciutto di Parma thin-sliced. Using the Plastic Wrap to help, roll the 12 slices prosciutto di Parma thin-sliced tightly around the beef, tucking in the ends like a burrito. Twist the ends of the Plastic Wrap tightly to form a compact log shape. Refrigerate for at least 30 minutes (or up to overnight). This resting step is critical — it helps the Wellington hold its shape when sliced.

Preheat the Oven to 425°F. Line a Baking Sheet with Parchment Paper. On a lightly floured surface, unfold and roll the 1 package puff pastry thawed if frozen, kept cold out to a rectangle large enough to fully wrap the beef log with a 2-inch overlap on all sides.

Whisk together the 2 eggs, 1 tbsp whole milk, and pinch of 1 tsp kosher salt to make the egg wash. Brush the 1 package puff pastry thawed if frozen, kept cold lightly with egg wash. Remove the 2 lb center-cut beef tenderloin trimmed and tied if uneven from the Plastic Wrap and place it at the bottom edge of the 1 package puff pastry thawed if frozen, kept cold. Roll it up snugly, pressing the seam firmly to seal. Fold and press the ends under to seal completely. Place seam-side down on the prepared Baking Sheet.

Brush the entire surface of the 1 package puff pastry thawed if frozen, kept cold generously with egg wash. Use the back of a Knife to score a light crosshatch or diagonal pattern on top — do not cut all the way through. Sprinkle lightly with flaky 1 tsp kosher salt if desired. Refrigerate the assembled Wellington for 15 minutes before baking to help the 1 package puff pastry thawed if frozen, kept cold stay flaky.

Bake at 425°F for 25–30 minutes, until the 1 package puff pastry thawed if frozen, kept cold is deep golden brown and an Instant-Read Thermometer inserted into the center of the 2 lb center-cut beef tenderloin trimmed and tied if uneven reads 125°F for medium-rare (the temperature will rise 5–10°F as it rests). For medium, pull at 130–135°F.

Transfer the Wellington to a cutting board and rest, uncovered, for 10 minutes before slicing. This is non-negotiable — resting allows the juices to redistribute and the 1 package puff pastry thawed if frozen, kept cold to set, giving you clean, beautiful slices. Slice with a sharp serrated Knife into 1.5-inch rounds and serve immediately.