First, make the duxelles. Combine the Cremini Mushroom (1 pound), Shallot (1), and Garlic (3 clove) in a food processor and pulse it all together until it is a fine mixture. Set it aside.
Heat the Olive Oil (2 dash) in a large skillet with deep sides. While it heats up, use kitchen twine to tightly tie up the Center-Cut Beef Tenderloin (1.75 pound) if it isn't already.
Season it generously with Salt (to taste) and Freshly Ground Black Pepper (to taste) before you brown it on all sides in the pan. It should take 2-3 minutes on each side to develop a nice brown crust. Transfer it to a cutting board to rest.
Add the duxelles to the same pan as the beef was browned in. Let it all cook down until the moisture is gone and it becomes a dry, almost pasty mixture. It should take about 6-7 minutes. When the duxelles is done, take the pan off the heat and set it aside.
Cut the twine off of the beef tenderloin and brush it all over with the Dijon Mustard (1 tablespoon).
Lay out two large, overlapping sheets of plastic wrap on a clean surface and lay out the Ham (4 slice) in an overlapping layer on it. Then spread the duxelles in a thick, packed-in layer on the ham.
Place the tenderloin in the center of the ham layer, then use the plastic wrap to wrap it up tightly, twisting the ends. Put it in the refrigerator to chill for 15 minutes.
While it is chilling, lay out another layer of plastic wrap and place one of the Puff Pastry (2) in the middle of it. Unwrap the chilled tenderloin from the plastic and place it in the center of the puff pastry sheet. Wrap it up tightly, making sure to pinch the seam and the ends together well. Wrap it in the plastic wrap and chill it again for 15 minutes.
Preheat the oven to 400 degrees F (200 degrees C) and line a sheet tray with a silicone mat or parchment paper. Take the chilled beef wellington and place it on the sheet tray.
Use a fork to gently score lined patterns in the dough if you wish, and use the second sheet of puff pastry to cut out any shapes you like. This is all optional. If you do cut out shapes to decorate, place them on the beef wellington and gently press them into the puff pastry dough.
Generously brush the whole thing with Egg (1) and bake it for 35-40 minutes to achieve a medium to medium well beef with some pink in the middle.
Let it rest for 5 or so minutes, then slice and serve. Enjoy!