Set a wire rack on a rimmed baking sheet. Season beef with salt and pepper. Heat oil in a large saute pan over high heat until hot.. Brown beef on all sides, about 10 minutes. Transfer to wire rack until cool to the touch, about 10 minutes. Spread pate, then duxelles (a cooked mushroom mixture) over fillet.
On a lightly floured work surface, overlap 2 puff-pastry sheets by 1/2 inch. Roll out to a 12-by-20-inch rectangle. Place beef lengthwise in middle; wrap pastry around beef so long sides overlap. Place seam side down on a clean wire rack on the baking sheet, and tuck pastry ends underneath. Brush with egg. Refrigerate 30 minutes (or up to 4 hours).
See AlsoClassic Beef WellingtonPreheat oven to 400 degrees. Bake until pastry is golden brown, about 30 minutes. Tent with foil; bake until center of beef registers 130 degrees on an instant-read thermometer (inserted through bottom side) for rare, about 15 minutes more. Transfer to a clean wire rack set over another baking sheet. Let stand 20 minutes before carving.
Cook's Notes
This dish can be prepared through step 1 up to a day ahead; refrigerate, covered, and bring to room temperature before proceeding.
