1. Prepare the Beef Tenderloin:

• Trim and Tie: Remove any silverskin or excess fat from the beef tenderloin. Tie the tenderloin in several places with kitchen twine to create a more uniform shape and ensure even cooking. This also helps it maintain its shape during searing.
• Season Generously: Pat the beef tenderloin dry with paper towels. Season generously on all sides with salt and freshly ground black pepper. Don’t be shy with the seasoning – this is your chance to build flavor into the beef itself.
• Sear the Beef: Heat olive oil and butter in a large oven-safe skillet (cast iron is ideal) over high heat until shimmering hot. Carefully sear the beef tenderloin on all sides, including the ends, until nicely browned. This should take about 2-3 minutes per side. Searing creates a flavorful crust and helps to seal in the juices. Don’t cook the beef through at this stage; you are just browning the exterior.
• Cool the Beef: Remove the seared beef tenderloin from the skillet and place it on a plate to cool completely. Let it rest for at least 30 minutes, or even better, chill in the refrigerator for an hour or two. Cooling the beef is crucial before proceeding, as it prevents the duxelles and pastry from becoming soggy.
• Mustard Coating: Once the beef is cooled, brush it evenly on all sides with Dijon mustard. This adds a flavorful layer and helps the duxelles adhere to the beef.

2. Prepare the Duxelles (Mushroom Mixture):

• Clean and Chop Mushrooms: Thoroughly clean the mushrooms using a damp paper towel or a mushroom brush. Finely chop the mushrooms. The finer the chop, the better the texture of the duxelles.
• Sauté Aromatics: Melt butter in a large skillet over medium heat. Add finely diced shallots and cook until softened and translucent, about 3-4 minutes. Add minced garlic and cook for another minute until fragrant, being careful not to burn the garlic.
• Cook the Mushrooms: Add the chopped mushrooms to the skillet and cook over medium-high heat, stirring frequently. Initially, the mushrooms will release a lot of liquid. Continue cooking until all the liquid has evaporated and the mushrooms are starting to brown and caramelize. This process can take 15-20 minutes. Evaporating the moisture is key to a flavorful and non-soggy duxelles.
• Deglaze and Season: Pour in dry sherry or dry white wine and scrape up any browned bits from the bottom of the skillet. Cook until the wine has mostly evaporated. Stir in chopped fresh thyme and parsley. Season generously with salt and freshly ground black pepper to taste.
• Cool the Duxelles: Remove the duxelles from the heat and spread it out on a plate or baking sheet to cool completely. Cooling is essential before assembling the Wellington, as warm duxelles can make the puff pastry soggy and difficult to handle.

3. Prepare the Crêpes (Optional but Recommended):

• Make Crêpe Batter: In a bowl, whisk together flour and salt. In a separate bowl, whisk together eggs and milk. Gradually whisk the wet ingredients into the dry ingredients until a smooth batter forms. Let the batter rest for at least 30 minutes (or up to an hour) at room temperature. Resting allows the gluten to relax, resulting in more tender crêpes.
• Cook the Crêpes: Heat a lightly buttered crêpe pan or small non-stick skillet over medium heat. Pour a thin layer of batter onto the hot pan, tilting the pan to spread the batter evenly into a thin circle. Cook for about 1-2 minutes per side, or until lightly golden brown. Repeat with the remaining batter, stacking the cooked crêpes on a plate lined with parchment paper. You should aim for crêpes that are slightly larger than the circumference of your beef tenderloin.

4. Assemble the Beef Wellington:

• Lay Out Crêpes (Optional): If using crêpes, lay out a large sheet of plastic wrap on a clean surface. Arrange the crêpes in a slightly overlapping layer on the plastic wrap, creating a surface large enough to wrap the beef tenderloin. This crêpe layer acts as a moisture barrier, further preventing the puff pastry from becoming soggy.
• Spread Duxelles: Spread the cooled duxelles evenly over the crêpe layer (or directly onto the plastic wrap if not using crêpes), leaving a border of about 2 inches around the edges.
• Place Beef: Place the mustard-coated beef tenderloin in the center of the duxelles layer.
• Wrap Tightly: Using the plastic wrap, tightly wrap the duxelles around the beef tenderloin, forming a log shape. Twist the ends of the plastic wrap tightly to secure the package. Refrigerate the wrapped Wellington for at least 30 minutes, or up to 2 hours. Chilling at this stage helps the Wellington hold its shape and makes it easier to work with the puff pastry.

5. Encase in Puff Pastry and Bake:

• Preheat Oven: Preheat oven to 200°C (400°F).
• Prepare Puff Pastry: Lightly flour a clean work surface. Roll out the puff pastry into a large rectangle, large enough to fully enclose the beef Wellington log.
• Egg Wash: In a small bowl, whisk together egg and milk (or water) to make an egg wash.
• Wrap in Pastry: Remove the chilled Wellington log from the refrigerator and unwrap it from the plastic wrap. Place the Wellington log in the center of the puff pastry. Brush the edges of the pastry with egg wash. Bring the pastry up and over the Wellington, enclosing it completely. Trim off any excess pastry, leaving about an inch of overhang. Crimp the edges of the pastry to seal it tightly.
• Decorate (Optional): Use the pastry trimmings to create decorative shapes (leaves, lattice, etc.) to adorn the top of the Wellington. Attach the decorations with egg wash.
• Score Pastry: Lightly score the top of the pastry in a decorative pattern using a sharp knife. Be careful not to cut all the way through to the filling. Scoring allows steam to escape during baking and helps create a beautiful, even browning.
• Egg Wash Again: Brush the entire surface of the puff pastry, including decorations, with egg wash for a golden, glossy finish.
• Bake: Place the Beef Wellington on a baking sheet lined with parchment paper. Bake in the preheated oven for 25-35 minutes, or until the puff pastry is golden brown and cooked through. The internal temperature of the beef should reach your desired level of doneness (see note below).
• Rest: Remove the Beef Wellington from the oven and let it rest for at least 10-15 minutes before slicing and serving. Resting allows the juices to redistribute throughout the beef, resulting in a more tender and flavorful Wellington.