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 black pepper freshly ground.

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 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 black pepper freshly ground.

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 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 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 kosher salt to make the egg wash. Brush the 1 package puff pastry thawed if frozen 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 puff pastry. 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 puff pastry 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 stay flaky.

Bake at 425°F for 25–30 minutes, until the 1 package puff pastry thawed if frozen is deep golden brown and an instant-read thermometer inserted into the center of the 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 to set, giving you clean, beautiful slices. Slice with a sharp serrated knife into 1.5-inch rounds and serve immediately.