• Step 1: Preheat your oven to 400°F (200°C). Season the beef tenderloin all over with salt and pepper.
• Step 2: Heat the olive oil in a large skillet over high heat. Sear the beef on all sides until a nice brown crust forms, about 2-3 minutes per side. Remove from the heat and let cool completely.
• Step 3: In the same skillet, sauté the mushrooms, garlic, and shallot over medium heat until the mushrooms are golden brown and the liquid has evaporated, about 5-7 minutes. Deglaze the pan with the brandy, if using, and cook until the liquid has reduced by half. Transfer the mushroom mixture to a food processor and pulse until it forms a smooth paste (the duxelle). Season with salt and pepper to taste.
• Step 4: On a lightly floured surface, roll out the puff pastry into a rectangle large enough to fully wrap the beef tenderloin. Spread the mushroom duxelle evenly over the pastry, leaving a 1-inch border.
• Step 5: Place the seared beef tenderloin in the center of the pastry. Fold the pastry over the beef, pressing to seal the seam and edges. Flip the wrapped tenderloin seam-side down and use a sharp knife to trim any excess pastry.
• Step 6: Transfer the Beef Wellington to a parchment-lined baking sheet. Brush the top and sides with the egg wash.
• Step 7: Bake for 40-50 minutes, or until the pastry is golden brown and the beef reaches your desired doneness (125°F for medium-rare, 135°F for medium).
• Step 8: Remove from the oven and let rest for 10-15 minutes before slicing and serving.