Boil potatoes until they can be pierced through with a fork. Drain and mash with a masher until smooth. If you want them REALLY smooth, peel off skin and put potatoes through a ricer.
 
While potatoes are boiling, melt butter in a sauté pan over medium heat. Add onions, sage and prosciutto. Sauté for about 5 minutes, remove from heat and set aside

 
In a bowl, add grated cheese and parsley to milk or half-and-half; set aside
 
Once potatoes are mashed, put them back into pot and set on stove over low heat. Add in prosciutto mixture and combine. Fold in mozzarella cheese and salt and pepper to taste. Add in milk/half-and-half mixture and blend well.
 
For a creamier mashed potato, add in milk or heavy cream a little at a time folding in until desired consistency is reached.