Tightly wrap the beef tenderloin in cling film and refrigerate to set the shape.
Heat some olive oil in a pan and sear the beef on all sides for 30-60 seconds until browned.
After searing, let the beef cool, then brush it with English mustard.
Overlap two pieces of cling film on a kitchen surface and lay prosciutto slices on top.
Spread the mushroom cream mixture over the prosciutto.
Place the beef on top and use the cling film to roll it into a tight barrel shape.
Chill the beef log in the refrigerator.
On a floured surface, roll out the puff pastry.
Unwrap the beef from the cling film and place it in the center of the pastry.
Fold the pastry over the beef to cover it completely and seal the edges.
Brush the pastry all over with the egg wash for a golden finish.
Bake in a preheated oven at 220C for around 20 minutes until the pastry is golden-brown.