Ingredients:

• 12-pound beef tenderloin, trimmed
• Salt and black pepper, to taste
• 2 tablespoons olive oil
• 2 tablespoons Dijon mustard
• 8 slices prosciutto
• 1/2 cup mushroom duxelles (finely chopped mushrooms cooked with shallots, garlic, and herbs)
• 1 sheet puff pastry, thawed if frozen
• 1 egg, beaten (for egg wash)

Instructions:

1. Preheat your oven to 425°F (220°C).
2. Season the beef tenderloin generously with salt and black pepper on all sides.
3. Heat olive oil in a large skillet over high heat. Sear the beef tenderloin on all sides until well-browned, about 2 minutes per side. Remove from the skillet and let it cool slightly.
4. Brush the seared beef tenderloin with Dijon mustard on all sides.
5. Lay out a large piece of plastic wrap and arrange the slices of prosciutto on it, overlapping slightly to form a large rectangle.
6. Spread the mushroom duxelles evenly over the prosciutto.
7. Place the beef tenderloin on top of the mushroom duxelles and carefully roll it up tightly using the plastic wrap. Twist the ends of the plastic wrap to secure the beef tenderloin in a tight cylinder shape. Refrigerate for 15-20 minutes to firm up.
8. Roll out the puff pastry on a lightly floured surface into a large rectangle, large enough to wrap around the beef tenderloin.
9. Remove the plastic wrap from the beef tenderloin and place it in the center of the puff pastry.
10. Brush the edges of the puff pastry with beaten egg.
11. Carefully fold the puff pastry over the beef tenderloin, sealing the edges tightly. Trim any excess pastry if necessary.
12. Place the wrapped beef tenderloin seam side down on a baking sheet lined with parchment paper.
13. Brush the entire surface of the puff pastry with the remaining beaten egg.
14. Using a sharp knife, make shallow diagonal cuts across the top of the puff pastry to create a decorative pattern.
15. Bake in the preheated oven for 25-30 minutes, or until the puff pastry is golden brown and crispy, and the beef tenderloin reaches your desired level of doneness (for medium-rare, the internal temperature should reach 130°F or 54°C).
16. Remove from the oven and let it rest for 10 minutes before slicing.
17. Slice the Beef Wellington into thick slices and serve immediately.