To make Beef Wellington, take a freshly cut tender beef tenderloin and tie it in four places using kitchen twine. Now season it generously with salt and pepper. Note that for every pound of meat you will need a teaspoon of salt and pepper. 

Now, coat the bottom of a heavy skillet with olive oil and keep it over high heat. Once done, take the beef tenderloin and place it in the pan, which may be nearly smoking. Sear it well until it becomes well-browned all over, including the ends. Keep changing the position of the tenderloin from side to side. It should not take more than 12 minutes. Now transfer it onto a plate and allow it to cool. When it has cooled down, cut off the twine and coat the tenderloin entirely with mustard.  Once done, keep it in the refrigerator to cool. 

In the meantime, start preparing the duxelles. Pulse mushrooms, shallots, thyme or parsley in a food processor until it is finely chopped.

Add butter to a skillet and let it melt over medium heat. Ensure the butter doesn't burn. Add the mushroom mixture to it and let it cook until the liquid has properly evaporated. This should ideally not take more than 25 minutes. This step is important as you do not want your pastry bottom to get soggy if the mushrooms let out water later while they get baked. Once done, season it with salt and pepper and allow it to cool in the refrigerator. 

Now start working on the herbed crepes. Whisk together eggs, flour, salt, sugar, and chives. Thereafter, add milk, water, a tablespoon of butter and whisk it together until it is smooth and properly blended. Your crepe batter is ready.

Now, over medium heat, place a crepe pan or a large skillet. Add olive oil and 1/4th cup of the whisked mixture. Once done, rotate the pan over the heat. This will cause the batter to spread evenly over the surface. Gradually, you will see the edges of the crepe begin to curl as it cooks. Note that the crepe cooks quite quickly. It does not take more than 45 to 60 seconds. Once done, flip the crepe to let the other side cook. Once cooked, the sides of the crepe will become pale golden brown in color. Now, take the pan down from the heat. Add olive oil again to it and continue with the procedure till you are done with the batter. Set the crepes aside.

Place a plastic wrap over a work surface. Ensure that it is double the length and width of the beef tenderloin. In the middle of the plastic wrap, evenly spread the crepes. Spread the duxelles evenly on the crepes. Place the prosciutto over the duxelles in a rectangle that is more than sufficient in size to cover the entire tenderloin and add more duxelles and evenly spread it over. 

Preheat the oven to 425° F. In the meantime, season the beef tenderloin properly and place it at the bottom of the duxelles-covered prosciutto. Once done, carefully roll the meat into the prosciutto-mushroom mixture using the plastic wrap and tightly fasten it by tucking the ends. Once rolled, twist the ends of the plastic wrap and tightly wrap it into a log before you keep it in the refrigerator to cool. This helps to maintain its shape and structure without allowing the layers to leak or fall apart. 

After some time, lightly apply flour on the work surface and spread out the puff pastry. Roll it into a rectangle - one that will cover the beef tenderloin perfectly. Ensure that it is slightly bigger than the prosciutto cover. Now, remove the tenderloin from the refrigerator. Take out the plastic wrap and place the tenderloin on the bottom of the puff pastry. Tightly roll the beef into the pastry but not before you brush the other three edges of the pastry with egg wash.

Once the entire tenderloin is covered properly in a puff pastry, trim any extras that you get. Use a fork to seal well and ruffle the edges. Again wrap the log tightly into a plastic wrap and allow it to chill for 20 minutes in the refrigerator. Thereafter, take out the plastic wrap and use a foil-lined baking sheet to transfer the tenderloin. Apply egg wash on the sheet and sprinkle it with salt.