1. Preheat your oven to 200°C and position a rack in the center. Melt the butter in a large skillet over medium heat. Add the chopped onion and cook, stirring often, until softened, about 5 minutes.
    

2. Incorporate the mushrooms into the skillet and cook, stirring occasionally, until they soften and release their moisture. Continue cooking until the liquid evaporates, which should take around 15 minutes.
    

3. Stir in the garlic and parsley, then season with salt and pepper to taste. Remove the skillet from heat and allow the mixture to cool to room temperature.
    

4. Once cooled, mix the pâté into the mushroom mixture thoroughly and set aside.
    

5. On a lightly floured surface, roll out the puff pastry into a sheet large enough to fully enclose the beef. Trim the edges and set aside the scraps for later use.
    

6. Spread the pâté mixture over the pastry, leaving a 2.5 cm border around the edges. Place the chilled beef tenderloin in the center of the pâté layer.
    

7. Brush the edges of the pastry with the beaten egg. Fold the pastry over the beef to create a tight parcel, sealing the edges securely.
    

8. Place the wrapped beef on a rimmed baking sheet with the seam-side down. Use the reserved pastry scraps to cut out decorative shapes and place them on top of the parcel. Brush the entire surface with the beaten egg.
    

9. Bake in the preheated oven for 40 to 45 minutes, or until the pastry is puffed and golden. The internal temperature of the beef should reach 52°C to 57°C for medium-rare, or 66°C for medium.
    

10. Remove from the oven and let the Wellington rest on a cutting board for 10 minutes.
     

11. Slice into thick portions and serve.
     

Why Let the Wellington Rest?

Resting the beef after baking allows the muscle fibers to relax and the juices to redistribute, ensuring that each slice of your Beef Wellington remains juicy, tender, and flavorful.