Peel potatoes and cut in half. Place in a large saucepan and cover with water.
Cover saucepan and bring to a boil over high heat. Then, reduce heat to medium and boil gently, partially covered, until potatoes are tender, from 20 to 30 minutes.
Drain potatoes and mash, preferably with a potato masher.
Stir in butter, sour cream, salt and pepper.
Stir in cheese just until mixed.
Pile into a heated serving dish.
Potatoes can be covered and kept warm in oven for up to 30 minutes.
Serve sprinkled with chopped parsley or finely sliced green onions, if you wish.