1. In a large pot, cover the rutabagas and potatoes with 2 inches of water and bring to a boil. Add a large pinch of salt and boil over moderately high heat until the vegetables are tender, about 50 minutes. Drain the rutabagas and potatoes and return them to the pot.
2. Shake the pot over high heat for about 2 minutes to dry the rutabagas and potatoes. Mash with a potato masher, then mash in the milk, cream and butter until a coarse puree forms. Season the puree with salt and pepper and serve hot.
Make Ahead: The recipe can be prepared up to 4 hours ahead.