Place the potatoes in a large saucepan and cover with cold water. Add ½ tablespoon of the salt and bring to a boil. Cook uncovered until tender, about 25 minutes. Drain well. Mash or rice the potatoes in a mixing bowl. Add the butter, milk, whipping cream, white pepper, nutmeg, and the remaining salt. Cover and keep warm in a double boiler until ready to serve.