Peel and thinly slice potatoes. Place in baking dish sprayed with nonstick spray. Add salt and pepper.
In a separate mixing bowl, stir sour cream, cream cheese and 2 cups white cheddar.
Wash and chop scallions. Rip apart the prosciutto into small pieces.
Gently fold in scallions and prosciutto into cream cheese mixture and then stir into potatoes until fully incorporated. To thin out mixture a bit, add up to ¼ cup milk.
Bake at 400 degrees for an hour, stirring once or twice. When potatoes can be pierced easily with a fork and are golden on the edges they are done! Sprinkle with a bit more chopped scallions and serve!