Preheat the oven to 375 degrees Fahrenheit. Take each piece of prosciutto one at a time and arrange in a muffin cup. Arrange each piece so that it covers the base and sides of the muffin cup.
Finely dice the onion, mushrooms and celery and roughly chop the walnuts and cranberries. Melt the butter in a skillet over a medium heat. Add the onion, mushrooms, celery, walnuts, cranberries, pancetta and sage. Stir well to combine. Pan-fry the mixture until the pancetta is cooked through and browned all over, and the vegetables are tender.
Divide the stuffing mixture evenly between the prosciutto cups. Each cup should be about 3/4 full. Sprinkle each stuffing cup with a half teaspoon of grated parmesan.
Transfer the tray to the oven. Bake for 12-15 minutes or until the prosciutto is crisp and golden, and the stuffing mix is hot through. Set aside to cool a little in the tin, then gently ease out of the muffin tray with a spatula to serve.