Prepare the Mushroom Duxelles: Heat a medium sauté pan over medium-high heat and add olive oil. Once hot, add the finely diced portobello mushrooms. Season generously with salt and pepper, then stir in thyme. Cook until the moisture has evaporated, about 10 minutes, and let cool.

Whisk the Chive Crepes: In a large mixing bowl, combine the eggs, flour, milk, chopped chives, thyme, and salt. Whisk until the batter is smooth. Let it rest in the refrigerator for at least 30 minutes.

Cook the Crepes: Spray a nonstick pan with cooking spray and heat on low. Pour 1/4 cup of the crepe batter into the pan, swirled to form a thin pancake. Cook for 25 to 30 seconds until set, then flip and cook briefly on the other side. Repeat until you've made about 5 crepes, placing parchment in between.

Sear the Beef Tenderloin: Season the beef tenderloin with salt and pepper. In a hot cast iron skillet with grapeseed oil, sear the beef for one minute on each side. Remove, drizzle with pan juices, brush with Dijon mustard, and let rest.

Assemble the Beef Wellington: Layer three pieces of plastic wrap on a cutting board. Cut two to three crepes into squares matching the beef’s length and lay them vertically. Layer prosciutto, spread mushroom duxelles, season, and pour the resting juices over the top. Place the beef at the base and roll tightly using the plastic wrap, then refrigerate for 20 minutes.

Wrap and Bake the Wellington: Preheat your convection oven to 425°F, or 450°F for non-convection. Roll the puff pastry to 1/8-inch thick on a moistened cutting board. Place the beef at the edge of the pastry, brush with egg wash, season, and tightly roll it up. Smooth and seal the seam, then brush with more egg wash, score the top, and place it on a parchment-lined tray. Chill for 5 minutes before baking for 18 to 20 minutes until golden brown. Let rest for 5 minutes before slicing and serving.