**Crêpe Preparation (If Using):** In a blender, combine flour, milk, eggs, melted butter, and salt. Blend until smooth. Heat a lightly oiled griddle or crêpe pan over medium heat. Pour about 1/4 cup of batter onto the hot pan, tilting to spread evenly. Cook for 1-2 minutes per side, or until lightly golden. Stack the crêpes and set aside to cool.

**Duxelles Preparation:** In a large skillet, melt butter over medium heat. Add shallots and garlic and cook until softened, about 3 minutes. Add chopped mushrooms and cook, stirring frequently, until all moisture has evaporated and the mushrooms are browned, about 15-20 minutes. Deglaze the pan with sherry or Madeira wine, scraping up any browned bits from the bottom. Stir in thyme, season with salt and pepper to taste. Remove from heat and let cool completely.

**Beef Preparation:** Preheat oven to 425°F (220°C). Pat the beef tenderloin dry with paper towels. Season generously with salt and pepper. Heat olive oil in a large oven-safe skillet over high heat. Sear the beef on all sides until deeply browned, about 2-3 minutes per side. Remove the beef from the skillet and let it cool slightly.

**Assembly:** Spread Dijon mustard evenly over the seared beef. Lay out plastic wrap on a clean surface. Arrange the cooled crêpes (if using) in a slightly overlapping pattern to create a sheet large enough to wrap the beef. Spread the cooled duxelles evenly over the crêpes. Place the mustard-coated beef on top of the duxelles. Use the plastic wrap to tightly roll the duxelles around the beef, forming a log. Twist the ends of the plastic wrap to secure it. Refrigerate the wrapped beef for at least 30 minutes (or up to overnight) to allow it to firm up. On a lightly floured surface, roll out the puff pastry to a rectangle large enough to encase the beef. Remove the beef from the refrigerator and unwrap it. Place the beef in the center of the puff pastry. Fold the pastry over the beef, trimming any excess. Crimp the edges to seal tightly. Place the Wellington seam-side down on a baking sheet lined with parchment paper. Brush the entire Wellington with egg wash. Use a sharp knife to score the top of the pastry in a decorative pattern. This allows steam to escape.

**Baking:** Bake in the preheated oven for 30-40 minutes, or until the pastry is golden brown and the internal temperature of the beef reaches your desired doneness (125°F/52°C for rare, 130°F/54°C for medium-rare, 135°F/57°C for medium). Remove from the oven and let rest for at least 10 minutes before slicing and serving.