Prepare mushroom duxelles: Finely chop mushrooms, shallots, garlic, and thyme. Slowly sauté mushrooms in butter without browning for about 12-14 minutes until they release and reduce their moisture. Add shallots, garlic, and thyme and cook until shallots are translucent but not colored.
Prepare beef tenderloin: Clean the tenderloin of sinew if needed. Season with salt and pepper. Let rest at room temperature for at least 20 minutes. Brown the tenderloin on all sides in butter to develop a crust, then let it rest.
Make pancake batter and cook pancakes: Whisk together eggs, flour, and milk until smooth and light. Cook thin pancakes in butter on a hot pan, flipping once, until golden but not crispy.
Make red wine glaze: Reduce half a bottle of red wine by half in a saucepan over medium heat. Add 1 liter of stock and reduce again. Stir in 1.5 packs of butter and let the sauce thicken and become glossy.
Assemble the Beef Wellington: Roll out puff pastry sheets carefully. Lay pancakes on the pastry, then Parma ham slices on top. Spread the mushroom duxelles evenly over the ham. Place the rested tenderloin in the center. Brush the tenderloin with Dijon mustard before placing it. Roll the pastry tightly around the filling, trimming excess dough. Optionally, create decorative patterns with pastry scraps and brush the entire surface with egg yolk to ensure browning and sealing.
Bake the Wellington: Preheat oven or air fryer. Bake in an air fryer at 180°C (356°F) for about 40 minutes, or in a convection oven at 200-220°C (392-428°F) for 40-50 minutes until the pastry is golden and cooked through.
Rest and serve: Let the Beef Wellington rest for 15 minutes before slicing to allow juices to redistribute. Serve with the red wine glaze and sautéed broccolini with peas, shallots, bay leaves, butter, salt, pepper, and a splash of lemon juice. Garnish with ramson flowers or wild garlic flowers for a fresh garlic aroma.