Heat a cast iron pan on high heat with 2 tbsp of oil. Season the tenderloin with salt and pepper and sear the beef on all sides until golden brown. We're not looking to cook the meat through because we're going to finish cooking it in the oven later on. Set the meat aside to cool.

To prepare the duxelles, heat a large skillet on medium high heat with 3 tbsp of oil and 2 tbsp of butter. Do this process twice if your pan is not large enough to cook the mushrooms. Make sure to not overcrowd the mushrooms. Add the shallots, thyme, and garlic and the mushrooms. Cook over high heat until the mushrooms are golden and dry. Finally, season with salt and pepper and set aside to cool.

Once the mushrooms have cooled enough to handle, remove the garlic and thyme and finely chop the cooked mushrooms.

Preheat the oven to 425F. Lightly flour your work surface and lay out the prepared puff pastry. The 14oz Dufour puff pastry was a perfect fit for my tenderloin but make sure the tenderloin fits properly on the puff pastry. The puff pastry should be 1/4" thick.

Spread the finely chopped mushrooms on the puff pastry while leaving a 1" - 1 1/2" border. Brush the edges with egg wash. Spread the mustard over the seared tenderloin and transfer it onto the mushrooms on the puff pastry. Tightly roll the pastry over the tenderloin, twist both ends to seal and trim off any excess. Tightly wrap with plastic wrap and refrigerate for 30 minutes.

Take off the plastic wrap and place the wellington on a parchment lined sheet pan with the seam side down. Brush with the egg wash and lightly sprinkle malden or coarse sea salt over the top. Bake for 20 minutes or until the internal temperature reaches 125F.

Let the beef rest for 10 minutes before slicing.