In a skillet, melt butter. Add leeks and mushrooms and sauté on medium high heat until leeks become tender, about 5-7 minutes. Add broth and stuffing mix. Mix well & remove from heat. Allow to cool to room temperature.
Roll out puff pastry to a rectangle large enough to cover turkey breast. 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. Brush turkey with mustard, then lay turkey just to right side of the center.
Bring right side edge up & over the top of the turkey breast. Repeat with the left side. You should trim any excess dough before sealing so it won’t be too thick.
Mix beaten egg with a Tbsp of water & brush edges of pastry to seal. Tuck the ends of the dough around the turkey breast & 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 & bake for about 30 minutes. Remove foil & continue baking until the crust is golden for about 20-30 minutes, depending on size of turkey breast. The internal temperature should reach 165 F.
Remove from oven and allow to rest for 5-10 minutes before slicing. Top with cranberry sauce and serve.