Trim and tie a beef tenderloin
Season the beef with salt and pepper
Sear the beef in a hot pan with oil on all sides
Brush the beef with mustard
Cool the beef completely
Finely chop mushrooms, shallots, and garlic
Cook the mushroom mixture until all moisture evaporates
Season the mushroom mixture and cool it
Lay out plastic wrap and arrange prosciutto slices in an overlapping layer
Spread the mushroom mixture over the prosciutto
Place the cooled beef on top
Roll tightly into a log and chill
Roll out puff pastry
Wrap the chilled beef log in puff pastry
Seal the edges and trim excess pastry
Brush the pastry with egg wash
Score the top lightly
Bake until the pastry is golden and the beef reaches desired doneness
Rest before slicing
Serve in thick slices