Slice the meat into 1/4 inch pieces. Carefully wash and chop the leeks, discarding the top few inches (the toughest green part). Peel and mince the garlic. Slowly sauté these ingredients in 1 tablespoon butter. Place in skillet on low heat for 5-10 minutes, then on medium-high until golden and crisp. (Add garlic last and do not allow to brown).
Wash, peel and slice potatoes into quarters. In a saucepan cover potatoes with water and bring to a boil. Add 2 teaspoons salt to water and reduce heat to a simmer, continuing to cook until tender (about 40 minutes). Drain and return potatoes to saucepan, mash potatoes in the still-hot pan while adding chunks of the butter.
When the butter melts, stir in the sour cream. Pour in the cream and milk; add salt and pepper to taste. Sprinkle with chopped chives and/or scallions, and optional cheese, if desired.
