Peel the rutabaga and sweet potatoes. Cut both in half, then into 2-inch chunks.
Place the rutabaga in a saucepan and the potatoes in another saucepan. Cover both with cold water and add salt. Bring to the boil and cook until tender. The potatoes will require less cooking than the rutabaga. Drain well.
While hot, put the vegetables through a food mill into a mixing bowl. Stir in salt and pepper to taste, 2tablespoons butter, nutmeg, and cream.
Spoon the mixture into a baking dish, dot with remaining butter, and bake20 minutes. At the last minute, run the dish under the broiler to glaze.