Slice potatoes and turnips 1/4-inch thick. Cook in boiling water for 15 minutes or until fork-tender. Drain. Whip unpeeled cooked potatoes and turnips with electric mixer, mixing until moderately smooth (don't overbeat them; a few lumps are nice). Add hot cream, butter, and sour cream. Season with salt and pepper, to taste. Whip again until blended. Adjust thickness by adding more cream, if desired.