Let tenderloin reach room temperature.
In a pan over medium-high heat with oil, place dried tenderloin, seasoned with salt, pepper and thyme. Sear the beef all around but not cook through.
While the beef is still hot spread Dijon mustard all around and set aside to cool down.
Finely chop the mushrooms, shallots and garlic.
In a non stick cold pan, over medium heat, cook the chopped vegetables and add a pinch of salt. Cook untill a dry paste forms and let it cool. This is your Duxel paste.
Place a cellophane sheet on your table, then spread the cooled Duxel. Then place your tenderloin in the center of the Duxel and roll the cellophane and tighten the sides. Place this in the fridge for 30min-1hour.
Preheat your oven at 220degrees Celcius.
On your table dust some flour. Then place your block of pastry and roll it out to a big square, to around ⅓ cm. Then unwrap the beef and place onto the bottom of your pastry leaving a 3cm border.
Then mix your egg and water to make an egg wash. Using a brush spread the eggwash along the border of your pastry.
Roll up the pastry from bottom to have some overlap at the edge. Then fold the sides and cut the excess.
Egg wash the surface of your pastry. Then score with a fork for a more interacte look and add some sea salt on top.
Place in a tray lined with baking paper and put in oven to cook for around 30-40 min. Till golden brown.
Take it out of the oven and let the Wellington rest for 10-20 min before cutting and enjoy