Prepare the beef tenderloin:

1. Using kitchen twine, tie tenderloin in 3 places and season with kosher salt and pepper.
2. Add olive oil to a frying pan, enough to coat the bottom. Over medium-high heat sear the tenderloin on all sides, including the ends, for about 2 minutes per side until well browned.
3. Transfer to a plate, **important: remove the kitchen twice** and let cool while you prepare the mushroom duxelle.

Prepare the mushroom duxelle:

1. Finely dice mushrooms, shallots, garlic cloves and thyme OR add to a food processor to finely dice.
2. To a frying pan add salted butter and olive oil, turn to medium heat and add the mushroom mixture.
3. Cook for about 20-25 minutes, until all of the liquid has evaporated. Season with salt and pepper to taste, then put in the fridge until ready to use.

To assemble the beef wellington:

1. Place two sheets of plastic wrap down on a cutting board, overlapping them so it’s at least 2 inches taller and wider than the puff pastry layer.
2. Layer the prosciutto in two layers of 4 slices so it’s large enough to cover the whole tenderloin, making sure all slices overlap horizontally and vertically – about a 12 inch long layer.
3. Spread the mushroom mixture evenly over the prosciutto layer.
4. Spread 2 tbsp dijon mustard over the seared beef tenderloin, kitchen twine now removed. Then, roll the beef tenderloin in the prosciutto-mushroom mixture layer until fully covered, tucking the prosciutto into the ends as you roll and rolling the plastic wrap around the entire roast to create a log. Twist the ends of the plastic wrap to seal and place in the fridge for 15 minutes.
5. Next, roll out the puff pastry layer into an approximate 14 inch rectangle and use the beef tenderloin to measure out the width to ensure coverage when rolling. Brush the beaten egg along all of the edges of the puff pastry and save the rest of the beaten egg.
6. Preheat the oven to 425F, line a baking sheet with parchment paper and remove the beef tenderloin from the fridge after the 15 minutes is up.
7. Remove the beef tenderloin from the plastic wrap and place at the bottom of the puff pastry layer. Then roll the beef tightly into the pastry, ensuring the ends are covered in pastry before trimming any excess pastry dough.
8. Use your fingers to pinch or use a fork to crimp the pastry ends and long edge to seal the pastry.
9. Wrap the final product one more time in plastic wrap and place in the fridge for 15 minutes.
10. After 15 minutes remove the wrapped beef tenderloin from the fridge and remove the plastic wrap.
11. Place the beef tenderloin on a parchment lined baking sheet and brush with remaining beaten egg and sprinkle with kosher salt.
12. Bake until pastry is golden brown and a meat thermometer placed in the center registers 120-125F for medium rare, about 40-45 minutes.
13. Let rest 10 minutes before slicing and serving.