Coat the bottom of your cast iron and heat over high heat.
Once heated, sear every side of your backstrap until browned. Remove from pan and set aside to rest.
Lay a piece of plastic wrap done over your work surface. Create your prosciutto layer by slightly overlapping prosciutto pieces. This should be twice as tall as your backstrap and longer in order to fully enclose the meat.
Place half of your duxelles over the prosciutto.
Cover one side of your backstrap with mustard and place on top of the duxelles mustard side down. Brush top with more mustard and top with remaining duxelles.
Using the plastic wrap to help you, cover/ enclose the backstrap with the prosciutto.
Wrap it tight and wind the ends to keep it sealed and in the loafish shape.
Place in the fridge for 30 minutes.
Preheat the oven to 425°F.
On a floured surface, rollout the puff pastry sheet into a rectangular shape. Not too much. It just needs to extend past your backstrap.
Place the backstrap at the bottom of the rectangle leaving about an 1.5 inches from the edge.
Brush the edges of the puff pastry with egg.
Roll the puff pastry with the backstrap inside until it completely covered. Pinch ends together. Brush top with remaining egg yolk.
Bake for ~25 minutes or until it reaches 130-135°.
Let rest for 10 minutes. This is important.
Slice it up and enjoy!