Preheat oven to 375 degrees.

In a skillet, melt butter.  Add leeks and mushrooms and saute on medium high heat until leeks become tender, about 5-7 minutes.  Add broth and stuffing mix.  Mix well and remove from heat.  Allow to cool to room temperature.

Roll out puff pastry to a rectangle large enough to cover turkey.  If using a turkey breast, you may have to use both sheets of pastry.  If two sheets are required, brush water or egg wash on one short edge of pastry and press edge of second sheet onto the first sheet.  Press gently until seam is sealed.

When stuffing is cool, cover pastry with stuffing, leaving a one-inch edge all the way around.

Cover the stuffing layer with prosciutto or ham.

Brush turkey with mustard, then lay turkey just to right side of the center.

Bring right side edge up and over the top of the turkey.  Repeat with the left side.  It may be easier to roll the turkey with pastry to the left until the pastry covers both sides.  You should trim any excess dough before sealing so it won't be too thick.

Mix beaten egg with a tbsp of water and brush edges of pastry to seal.  Tuck the ends of the dough around the turkey and over each other to seal.

Place the turkey, seam side down, on a parchment covered baking sheet.  Cut slits in the top of the dough to vent steam.  Brush entire pastry with egg wash.

Tent a piece of foil over the top and bake in a 375 degree oven for about 30 minutes.  Remove foil and continue baking until the crust is golden and the internal temperature is 165 degrees 20-30 minutes depending on size of turkey.

Remove from oven and allow to rest for 5-10 minutes before slicing.

Top with cranberry sauce and serve.