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.
If you do not have an electric hand mixer, you can use a potato masher.