1. Prepare the Duxelles (Mushroom Mixture):

• Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a large skillet over medium heat.
• Add the finely chopped shallots and cook until softened and translucent, about 3-4 minutes.
• Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.
• Add the finely chopped mushrooms to the skillet. Season with salt and freshly ground black pepper.
• Cook the mushrooms, stirring occasionally, until they release their moisture and then begin to brown and dry out. This process will take about 15-20 minutes. The key is to cook out as much moisture as possible from the mushrooms to prevent a soggy Wellington.
• Pour in the dry sherry or dry white wine and deglaze the pan, scraping up any browned bits from the bottom.
• Continue to cook until the liquid has evaporated and the duxelles is relatively dry and paste-like.
• Stir in the chopped fresh thyme leaves.
• Taste the duxelles and adjust seasoning with salt and pepper as needed. If using truffle oil or paste, stir it in now, a little at a time, tasting as you go.
• Remove the duxelles from the skillet and spread it out on a plate or baking sheet to cool completely. Cooling the duxelles is important before assembling the Wellingtons.

2. Sear the Beef Tenderloin Steaks:

• Pat the beef tenderloin steaks dry with paper towels. Season generously on all sides with salt and freshly ground black pepper.
• Heat the remaining 1 tablespoon of olive oil and 1 tablespoon of butter in the same skillet over medium-high heat until hot and shimmering.
• Sear the beef steaks on all sides, including the top and bottom edges, for about 1-2 minutes per side, until nicely browned. The goal is to create a flavorful crust, not to cook the beef through at this stage.
• Remove the seared beef steaks from the skillet and place them on a plate to cool completely.

3. Assemble the Individual Beef Wellingtons:

• Once the beef steaks and duxelles are completely cooled, lightly brush each steak all over with Dijon mustard. This adds flavor and helps the duxelles adhere.
• Lay out a sheet of plastic wrap on a clean work surface. Place a slice of prosciutto on the plastic wrap, slightly larger than the size of a beef steak. If using pâté, spread a thin layer of pâté over the prosciutto slice.
• Spoon a generous amount of the cooled duxelles (about 1/4 to 1/3 cup) on top of the prosciutto, spreading it evenly to roughly the size and shape of the beef steak.
• Place a Dijon-mustard-coated beef steak in the center of the duxelles.
• Using the plastic wrap, tightly wrap the prosciutto and duxelles around the beef steak, forming a compact cylinder. Twist the ends of the plastic wrap to seal it tightly.
• Repeat this process with the remaining beef steaks, prosciutto, and duxelles.
• Place the wrapped Beef Wellingtons in the refrigerator and chill for at least 30 minutes, or up to 2 hours. Chilling helps them firm up and makes them easier to handle.

4. Prepare the Puff Pastry and Bake:

• Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
• Lightly flour a clean work surface.
• Unwrap the thawed puff pastry sheet. Depending on the size of your puff pastry sheet, you may need to cut it in half to make it easier to work with.
• Roll out the puff pastry slightly thinner, to about 1/8 inch thickness. You want it large enough to wrap around each beef Wellington.
• Cut the puff pastry into 4 equal squares, large enough to enclose each beef Wellington.
• Remove the chilled beef Wellingtons from the refrigerator and unwrap them from the plastic wrap.
• Place a beef Wellington in the center of a puff pastry square.
• Bring the corners of the puff pastry up and over the beef, enclosing it completely. Trim away any excess pastry.
• Press the edges of the pastry together to seal tightly. You can crimp the edges with a fork for a decorative touch and to ensure a good seal.
• Place the assembled Beef Wellingtons seam-down on the prepared baking sheet.
• Brush the top and sides of each Wellington generously with beaten egg wash.
• If desired, score the top of each Wellington lightly with a sharp knife in a decorative pattern (be careful not to cut all the way through the pastry). This helps steam escape during baking.
• Bake in the preheated oven for 20-25 minutes, or until the puff pastry is golden brown and puffed up. The internal temperature of the beef for medium-rare should be around 130-135°F (54-57°C). Use a meat thermometer inserted into the side of a Wellington to check the temperature.
• Remove the baked Beef Wellingtons from the oven and let them rest for 5-10 minutes before serving. Resting allows the juices to redistribute, resulting in a more tender and flavorful beef.

5. Serve and Enjoy:

• Serve the Individual Beef Wellingtons immediately while they are hot and the pastry is crisp.
• Garnish with fresh thyme sprigs or parsley, if desired.
• Slice each Wellington in half or serve whole.