For the beef tenderloin:

Season the beef tenderloin generously with kosher salt. Wrap the beef tightly with cling wrap into a cylinder shape and refrigerate for at least 1 hour (or overnight for best results).

Heat oil in a large pan over medium-high heat. Once hot, sear the beef on all sides, including the ends, until lightly golden. This step helps seal in the juices, but doesn’t fully cook the tenderloin.

Brush the seared beef with Dijon mustard all over and refrigerate until cooled or until needed.

For the duxelles:

Clean the mushrooms and trim the stems if needed. Finely chop the mushrooms by hand or using a food processor.

In a pan, melt butter over medium-high heat. Add chopped mushrooms, shallots, thyme, salt, and pepper. Cook the mixture, stirring frequently, until it becomes dry and crumbly (about 20 minutes).

Remove from heat and let the duxelles cool. You can make this step up to 2 days in advance.

For the spinach (optional):

Bring a pot of water to a boil. Prepare a bowl of ice water. Add spinach to the boiling water and cook until wilted. Drain and transfer immediately into ice water to cool.

Once cooled, drain the spinach well and squeeze out excess moisture. Set aside.

Assembly:

Lay cling wrap on your work surface. Spread out the spinach leaves to create a rectangle about 2 inches longer than the beef tenderloin. Overlap the spinach leaves to ensure it covers the entire tenderloin.

Place the cooled beef tenderloin at the center of the duxelles layer. Using the cling wrap, lift the spinach, and duxelles to wrap the beef into a tight log. Twist the ends of the cling wrap and refrigerate while preparing the puff pastry.

Puff Pastry Assembly:

Preheat the oven to 420°F (215°C). Beat together egg yolks with milk or water to create an egg wash.

On a lightly floured surface, roll out the puff pastry to a size large enough to completely wrap the beef log. The length should be about 3 inches longer than the beef and the width about three times the log’s width.

Take the beef log out from the fridge, remove the cling wrap, and place the log at one end of the puff pastry. Brush about a quarter of the pastry with egg wash, then roll the log in the puff pastry. Fold the ends of the pastry under the log to seal it.

If using lattice work, roll out additional puff pastry, cut it into strips, and weave a lattice over the beef Wellington. Brush the entire pastry with egg wash.

Bake:

Place the Beef Wellington on a parchment-lined baking sheet and refrigerate while preparing for baking.

Bake for about 40 minutes for rare, 45 minutes for medium-rare, and 50 minutes for medium doneness. After 20-25 minutes, loosely cover with foil to prevent the pastry from darkening too much.

Check the internal temperature with a kitchen thermometer: remove for rare at 105-108°F (40-42°C), medium-rare at 110-115°F (43-46°C), and medium at 115-120°F (46-49°C).

Let the Beef Wellington rest for 25-30 minutes before slicing. This helps the juices redistribute and ensures perfect doneness.