• Preheat the oven to 400°F (200°C) while you prepare the Beef Wellington.

Sear the Beef:

• Generously season the beef tenderloin with salt and pepper. Heat 2 tablespoons of olive oil in a large skillet over high heat. Sear the beef on all sides, letting each side cook for about 2 to 3 minutes until nicely browned. Avoid moving the beef too soon to allow for proper browning.

Brush with Mustard:

• Take the seared beef out of the skillet and allow it to cool slightly. Once it has cooled, brush it evenly on all sides with the mustard, ensuring it is thoroughly coated.

Prepare the Mushrooms:

• Finely chop the mushrooms using a food processor, pulsing until they are minced.
• In the same skillet, heat it over medium-high heat. Add the chopped mushrooms to the pan and cook, stirring occasionally, for 10 to 15 minutes until all the moisture has evaporated. Once the mushrooms are dry and have a concentrated flavor, remove them from the pan and set aside to cool.

Wrap the Beef in Mushroom Paste and Ham:

• Place a large piece of plastic wrap on a flat surface. Lay the slices of ham on the plastic wrap, slightly overlapping to create a sheet large enough to encase the beef. Spread the mushroom mixture evenly over the ham layer.
• Position the mustard-coated beef in the center, then roll the mushroom and ham layers tightly around the beef using the plastic wrap to guide you. Roll it into a firm, barrel shape, twisting the ends of the plastic wrap to secure. Chill in the refrigerator for 20 minutes.

Roll Out the Puff Pastry and Wrap the Beef:

• Lightly flour a flat surface and roll out the puff pastry to a size that will fully enclose the wrapped beef.
• Unwrap the beef from the plastic wrap and place it in the center of the puff pastry. Brush the pastry edges with the beaten egg yolks to help seal.
• Fold the pastry over the beef, trimming any excess to avoid overlapping more than two layers of pastry, as thick layers may not cook through. Place the wrapped beef seam-side down on a small plate and brush the pastry surface with more egg yolk. Chill for 5 to 10 minutes.

Brush with Egg Wash and Score:

• Transfer the pastry-wrapped beef to a baking sheet. Brush the top once more with egg yolk. Using a sharp knife, lightly score the surface of the pastry for decorative effect, taking care not to cut too deeply. Sprinkle flaky salt over the top.

Bake in the Oven:

• Bake the Beef Wellington in the preheated oven at 400°F (200°C) for 25 to 35 minutes, until the pastry is golden and crisp. For medium-rare beef, use an instant-read thermometer and pull the roast out of the oven when it reaches 125 to 130°F (52 to 54°C).
• Allow the Wellington to rest for 10 minutes before slicing into 1-inch thick pieces.
• Leftover slices can be stored in the refrigerator for up to 2 days. Wrap tightly in foil and reheat in a 350°F oven. Once warmed, remove the foil and bake briefly to re-crisp the pastry. Note that reheating may cook the beef further, bringing it closer to medium or well-done.