Remove the beef tenderloin from the refrigerator 30 minutes before cooking. Pat it completely dry with paper towels — this is essential for a proper sear. Season generously all over with salt and pepper.

Heat olive oil in a large heavy-bottomed skillet (cast iron preferred) over the highest heat possible until it is smoking. Add the beef and sear for 1-2 minutes per side, including the ends, until a deep mahogany crust forms all over. This should take about 4-5 minutes total. Do not move the beef while each side sears.

Transfer the seared beef to a plate. Brush it all over with English mustard while it is still warm. Transfer to the refrigerator uncovered and chill for at least 30 minutes, until completely cold.

Pulse the mushrooms in a food processor until very finely chopped — almost minced. Work in batches if necessary.

Melt butter in a large skillet over medium-high heat. Add the shallots and garlic and sauté for 2 minutes until softened. Add the chopped mushrooms and thyme leaves.

Cook, stirring frequently, for 15-20 minutes until all the moisture has completely evaporated and the mushrooms look dry and almost paste-like. If using sherry or wine, add it halfway through and let it cook off. Season with salt and pepper. Spread on a plate and refrigerate until completely cold, at least 20 minutes.

Lay a large sheet of plastic wrap on your work surface. Arrange the prosciutto slices in a rectangle, overlapping them slightly, large enough to completely wrap the beef — roughly 12x10 inches.

Spread the cold duxelles evenly over the prosciutto in a thin, even layer, leaving a 1-inch border around the edges.

Place the chilled, mustard-coated beef at the near edge of the prosciutto. Use the plastic wrap to roll the prosciutto tightly around the beef, twisting the ends of the plastic wrap to form a tight log shape. Refrigerate for at least 20 minutes to set its shape.

Roll out the puff pastry on a lightly floured surface. Brush the pastry with egg wash. Place the prosciutto-wrapped beef log at the near edge of the pastry and roll tightly, sealing the seam underneath and pressing the ends in firmly. Trim any excess pastry from the ends and crimp to seal.

Wrap the assembled Wellington tightly in plastic wrap and refrigerate for at least 30 minutes (or up to 24 hours). This resting period is crucial for the pastry to stay flaky and hold its shape during baking.

Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper and place it in the oven to preheat as well.

Remove the Wellington from the refrigerator and unwrap it. Brush generously all over with egg wash. Use the back of a knife to score a light decorative pattern on top (do not cut through). Sprinkle with flaky sea salt.

Place the Wellington seam-side down on the hot preheated baking sheet. Bake for 25-30 minutes until the pastry is deeply golden all over and an instant-read thermometer inserted into the center reads 125°F (52°C) for medium-rare.

Transfer the Wellington to a wire rack and rest uncovered for 10-15 minutes. Do not skip this step — it allows the juices to redistribute and the temperature to carry over to a perfect medium-rare. Slice with a sharp carving knife into 1.5-inch rounds and serve immediately with red wine jus.

While the Wellington bakes, sauté the sliced shallot in a small saucepan over medium heat for 2 minutes. Add the red wine and thyme and simmer until reduced by half, about 5 minutes.

Add the beef stock and Worcestershire sauce and continue to simmer until reduced by half again and lightly syrupy, about 10-12 minutes. Strain through a fine-mesh sieve, return to the pan, and swirl in the cold butter off the heat to finish. Season to taste. Keep warm until serving.