Rub flour, salt, and 50g butter cubes until the mixture looks like breadcrumbs.
Stir in 150ml cold water to form a soft dough and pat into a 2cm thick block.
Roll dough to 30x20cm and roll 250g butter block between greaseproof paper to 15x20cm.
Place butter on pastry, fold sides to cover it, and pinch edges to seal.
Fold in half lengthways, turn 90 degrees, and roll back to 30x20cm.
Repeat folding process and give another quarter-turn.
Repeat rolling one last time, wrap in film, and chill for 1 hour.
Preheat oven to 100°C.
Brown the beef fillet in a hot pan with oil on all sides.
Wrap beef tightly in foil layers and roast for 1 hour, then chill until cold.
Fry mushrooms and tarragon in butter until liquid evaporates.
Wilt spinach in the same pan with more butter and set aside.
Whisk 2 eggs, flour, and milk into a smooth pancake batter.
Fry thin pancakes in butter until golden on both sides.
Beat the chicken liver pâté until smooth.
Increase oven temperature to 230°C.
Roll puff pastry to 40x50cm and roughly 3mm thickness.
Layer pancakes over pastry, leaving a 2.5cm border.
Spread pâté over pancakes, then layer spinach, mushrooms, and the beef.
Brush borders with egg yolk, roll tightly, and glaze the exterior.
Bake for 20-25 minutes until pastry is golden-brown.
Sauté shallots in butter, deglaze with Madeira, add stock, and reduce by half.
Grill sliced ceps in oil for 2 minutes per side.
Carve the Wellington and serve with the Madeira sauce and grilled mushrooms.