Preheat your oven to 400°F (200°C).
Sear the beef tenderloin in olive oil for 3-4 minutes on each side until browned.
Remove from pan, season with salt and pepper, and let cool.
In the same pan, add the chopped mushrooms, cooking them until all moisture evaporates.
Stir in the Dijon mustard, mixing until well combined, then let cool.
Roll out the puff pastry on a floured surface to create a large rectangle.
Spread the mushroom mixture over the pastry, leaving edges clear for sealing.
Place the cooled, seared beef in the center of the pastry, optionally spreading pâté on top.
Fold the pastry over the beef, sealing the edges with the beaten egg.
Brush the top with egg wash for a golden finish and cut a few slits for steam.
Bake in the preheated oven for 25-30 minutes or until the pastry is golden brown.
Remove from the oven and let rest for 10-15 minutes before slicing.