1. Preheat the oven to 350°F. Peel, quarter, and boil the potatoes for about 15 minutes, or until just cooked. While potatoes cook, finely chop the chives and green onions, set aside. Drain the potatoes and return to the pot.
2. In a microwave-safe bowl, melt the butter. Add to the potatoes and mash coarsely. Add the sour cream, salt (in moderation), and pepper. Mash until smooth, taking care not to overwork the potatoes, or they will become starchy. Stir 3/4 of the chopped chives and green onions into the mashed potatoes, along with the shredded cheese. Spoon the mixture into a casserole dish. At this point, you can bake the casserole, or cover and chill it until later.
3. If using the prosciutto, chop and scatter the pieces over the potatoes. Bake casserole for 20 minutes (30 minutes if using straight from the refrigerator). Sprinkle the remaining green onions and chives over the casserole and serve.
Per serving: 339 calories, 19g fat, 12g saturated fat, 60mg cholesterol, 259mg sodium, 34g carbohydrate, 2g fiber, 3g sugar, 10g protein