Steps

make Mushroom Duxelle

pat the beef dry and season with salt and pepper. Sear on all sides in a pan over medium-high heat ~30s/side. Remove and let cool

brush the beef with mustard. Lay prosciutto on a piece of cling filk. Spread mushroom duxelles evenly over the prosciutto. Place beef on top and roll tightly using the cling film. Chill for 15 minutes

preheat the oven to 400°F

roll out puff pastry. Remove beef from cling film and place in the center of the pastry. Fold the pastry over beef, sealing the edges. Place seam-side down on a baking tray. Brush with egg yolk for a golden finish and decorate with pastry straps if desired

bake until pastry is golden and beef is medium rare (internal temp 130°F) ~15 minutes. Rest 5 minutes before serving