Preheat oven to 400℉. Line a large rimmed baking sheet with parchment paper.
Sprinkle some flour on a clean flat surface. Roll out puff pastry sheet to flatten the edges. Brush the entire surface with the Dijon mustard. Next, place your prosciutto slices on to the puff pastry and sprinkle evenly with cheese. Start rolling up the short side of the rectangle like you would for cinnamon rolls. Slice the dough into 1 inch thick rolls and transfer to your baking sheet. Brush the rolls tops and sides with the egg wash. Bake for 22-25 minutes or until golden brown on top. Serves best right away.