Prepare the Beef: Pat the beef tenderloin completely dry with paper towels. This is crucial for achieving a good sear. Season generously all over with 1 tbsp (15g) of salt and 1 tsp (5g) of freshly ground black pepper. Let the beef sit at room temperature for at least 30 minutes before searing to ensure even cooking.

Sear the Beef: Heat 2 tbsp (30ml) of olive oil in a large, heavy-bottomed skillet over medium-high heat until shimmering. Carefully place the seasoned beef tenderloin in the hot skillet and sear for 2-3 minutes per side, until a deep golden-brown crust forms all around. Do not overcrowd the pan; sear in batches if necessary. The goal is to create a beautiful crust, not to cook the interior.

Cool and Coat the Beef: Once seared, immediately remove the beef from the skillet and place it on a wire rack set over a baking sheet. Brush the entire surface of the beef evenly with 2 tbsp (30g) of Dijon mustard. Allow the beef to cool completely, about 20-30 minutes. This step is important to prevent the pastry from becoming soggy.

Make the Mushroom Duxelles: While the beef cools, prepare the duxelles. Finely chop the mushrooms in a food processor until they resemble coarse breadcrumbs. Be careful not to over-process into a paste. In the same skillet used for searing (or a clean one), melt 2 tbsp (30g) of butter over medium heat. Add the chopped shallots and cook until softened, about 3-4 minutes. Add the minced garlic and fresh thyme leaves, and cook for another minute until fragrant.

Cook Down the Duxelles: Add the finely chopped mushrooms to the skillet with the shallots, garlic, and thyme. Increase the heat to medium-high and cook, stirring frequently, until all the moisture released by the mushrooms has evaporated and the mixture is dry and paste-like. This can take 15-20 minutes. If using white wine, pour it in and cook until completely evaporated, scraping up any browned bits from the bottom of the pan. Season the duxelles with salt and pepper to taste.

Cool the Duxelles: Spread the mushroom duxelles mixture onto a plate or baking sheet and let it cool completely. This ensures it doesn't prematurely cook or melt the prosciutto and puff pastry.

Assemble the Wellington: Lay a large piece of plastic wrap on your work surface. Arrange the 12 slices of prosciutto on the plastic wrap, overlapping them slightly to form a rectangle large enough to encase the beef tenderloin. Spread the cooled mushroom duxelles evenly over the prosciutto layer, leaving a small border. Place the cooled, mustard-coated beef tenderloin at one end of the prosciutto-duxelles rectangle.

Wrap the Beef: Using the plastic wrap to help, carefully and tightly roll the prosciutto and duxelles around the beef tenderloin, forming a compact log. Twist the ends of the plastic wrap to secure the shape and refrigerate for at least 30 minutes (or up to 24 hours) to firm up. This helps maintain the shape during baking.

Prepare the Puff Pastry: Lightly flour your work surface. Unfold or roll out the puff pastry sheet (500g / 1.1 lb) into a rectangle large enough to fully enclose the beef log. If your pastry comes in a block, roll it out to about 3-4mm (1/8 inch) thickness. Trim any uneven edges.

Encase the Beef: Remove the beef log from the refrigerator and unwrap it from the plastic wrap. Place the beef log seam-side down in the center of the rolled-out puff pastry. Brush the edges of the pastry with the egg wash (2 egg yolks whisked with 1 tbsp (15ml) water). Carefully fold the pastry over the beef, trimming any excess pastry if necessary. Press the edges firmly to seal, ensuring there are no gaps. You can use any excess pastry to create decorative elements.

Chill the Wellington: Place the wrapped Wellington seam-side down on a baking sheet lined with parchment paper. Brush the entire surface evenly with the remaining egg wash. For a decorative finish, you can lightly score the top of the pastry with the back of a knife (avoid cutting all the way through) or add pastry cut-outs. Chill the Wellington in the refrigerator for at least 30 minutes to firm up the pastry and prevent shrinkage during baking.

Bake the Wellington: Preheat your oven to 200°C (400°F). Remove the chilled Wellington from the refrigerator. Sprinkle the top with flaky sea salt. Bake for 20 minutes at 200°C (400°F), then reduce the oven temperature to 180°C (350°F) and continue baking for another 15-20 minutes, or until the pastry is deep golden brown and puffed. The internal temperature of the beef should reach 50-52°C (122-125°F) for medium-rare.

Rest the Wellington: Once baked, carefully transfer the Beef Wellington to a cutting board. Let it rest for at least 10-15 minutes before slicing. This allows the juices to redistribute, ensuring a moist and tender result.

Slice and Serve: Using a sharp serrated knife, carefully slice the Beef Wellington into thick portions, about 2.5cm (1 inch) thick. Serve immediately with your favorite accompaniments, such as roasted vegetables, mashed potatoes, or a red wine reduction sauce.