Season the beef tenderloin generously with salt. Sear it in a hot pan with a little oil on all sides until a nice crust forms.
Remove the beef from the pan and immediately brush it all over with Dijon mustard while still warm. Wrap tightly in plastic wrap and chill in the fridge to set its shape.
To make the duxelles, sauté minced garlic and thyme in oil, then add finely chopped mushrooms. Cook until the moisture begins to evaporate, then add a splash of cognac. Flambé and continue cooking until completely dry. Spread onto a tray to cool.
In a blender, combine the spinach, one egg, flour, and milk. Blend until smooth to create a green crepe batter. Cook a thin, large crepe in a non-stick pan.
Lay out the cooked green crepe and spread the cooled mushroom duxelles evenly over it.
Unwrap the chilled beef tenderloin, place it on the duxelles-covered crepe, and roll it up tightly. Wrap the entire log in plastic wrap again to hold its shape.
Roll out a sheet of puff pastry. Unwrap the beef parcel and place it on the pastry. Wrap the pastry tightly around the beef, sealing the edges.
If desired, use a lattice cutter on additional puff pastry to create a decorative net and drape it over the top of the Wellington. Brush the entire pastry with a beaten egg wash and garnish with small thyme sprigs.
Bake in a preheated oven until the puff pastry is deeply golden and crispy. Let the Beef Wellington rest before slicing.
Cook it hands-free.
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