Preheat the oven to 100°C.
Briefly sear the beef fillet in hot olive oil until browned on all sides.
Wrap the beef tightly in several layers of cling film to form a cylinder.
Roast the wrapped beef for 1 hour, then chill in the fridge until cold.
Fry chopped Portobello mushrooms in butter until the liquid evaporates.
Wilt the spinach in a pan with butter and set aside.
Whisk two whole eggs, flour, and milk to create a smooth pancake batter.
Fry the thin pancakes in butter until golden-brown on both sides.
Beat the pâté until smooth and increase oven temperature to 230°C.
Roll puff pastry into a 40cm x 50cm rectangle.
Layer pancakes, pâté, spinach, and mushrooms onto the pastry, leaving a border.
Place the beef on top, brush the border with egg yolks, and roll tightly.
Brush the exterior with egg yolks and bake for 20 minutes.
Sauté wild mushrooms in butter and deglaze with Madeira.
Add concentrated stock and simmer until reduced by one-third.
Slice the Wellington and serve with the mushroom Madeira sauce.