Preheat the oven to 220°C.
Season the beef and sear in a hot pan with oil until browned on all sides.
Roast the beef for 15–18 minutes, then cool and chill for at least 1 hour.
Fry mushrooms in butter for 5–10 minutes and drain juices into a bowl.
Mix cooled mushrooms with Parmesan, tarragon, mustard, and egg yolk then chill.
Roll pastry into a 40cm square and place the beef and mushroom mixture on top.
Fold pastry to seal the beef and chill for 30 minutes while preheating the oven to 220°C.
Apply egg wash and a lattice of pastry strips over the Wellington.
Roast for 30–35 minutes until the pastry is golden and crisp.
Prepare the sauce by frying mushrooms and stirring in flour, crème fraîche, and seasonings.
Simmer the sauce for 3 minutes and stir in reserved meat juices.
Rest the beef for 15 minutes before carving and serving with sauce.