Place a medium sauté pan over medium high heat. Add oil and when hot, add mushrooms. Season well with salt and pepper, then add the thyme. Stirring frequently to be sure mushrooms don’t stick, cook until all moisture has evaporated, about 10 minutes. Remove from heat and allow to cool.

In a large mixing bowl, combine eggs, flour, milk, chives, thyme, and salt. Whisk until incorporated. Transfer to the refrigerator to rest for at least 30 minutes.

Apply a light coat of cooking spray to a 10-inch nonstick pan and place over low heat. Add 1/4-cup crepe batter and swirl the pan so that the batter spreads evenly along the bottom, forming a very thin pancake. Allow to cook for 25 to 30 seconds until set. Using an offset spatula to lift one side, flip crepe and cook on opposite side for just a few seconds, then transfer crepe to a plate. Move quickly: you don’t want any browning to occur. Continue cooking until batter is finished, making about 5 crepes. Place a parchment square between each crepe to keep them from sticking together.

Season beef tenderloin liberally with salt and pepper on all sides. Use your hands to rub salt and pepper into meat while rolling and shaping beef into a cylinder. Place cast iron skillet over medium high heat and add grapeseed oil. Once the pan is hot, sear the beef, cooking one minute per side. Remove beef to a sheet tray and pour any juices from the pan over it. While hot, brush Dijon mustard all over, and then let it rest.

Moisten your cutting board with a damp towel. Layer three pieces of plastic wrap on the board. Take two to three pancakes (depending on the width of your beef tenderloin) and trim into squares the length of the meat, then line them up vertically on the plastic. Layer the prosciutto slices on top of the crepes, then spread the mushroom duxelles over the prosciutto. Season with salt and pepper. Pour resting juices from the beef over the mushrooms, then place beef at the bottom of the crepe. Using the plastic wrap to lift as you pull the crepe over the beef, roll the beef up completely, smoothing plastic and twisting the ends to form a tight package. Refrigerate for 20 minutes.

Preheat convection oven to 425°F or non-convection to 450°F.

Moisten cutting board again, then layer three more pieces of plastic wrap. Place puff pastry in center, then use a rolling pin or dowel to evenly roll it to 1/8-inch thick, trimming if necessary to fit your tenderloin. Unwrap beef and place on the pastry 1/3 of the way up. Lightly brush pastry with egg wash and season with salt and pepper. Lift the plastic wrap up and roll the pastry over the beef until the edges meet, then massage the pastry, smoothing it evenly across and sealing the seam.

Twist the plastic, taking excess pastry with it, until pastry melts together, and cut off the ends. Unwrap the Wellington, discard the plastic and smooth the ends, then apply more egg wash. Use the back of a knife to gently score the top of your Beef Wellington and create a decorative pattern. Season with more salt, place on a parchment-lined tray, and insert thyme sprig. Refrigerate for 5 minutes.

Place in the center of the oven and bake for 18 to 20 minutes, until golden brown. Remove and let rest for 5 minutes before slicing and serving.