Put the rutabaga in a large saucepan and cover by about 2 inches of water. Season the water generously with salt.
Partially cover the pot and bring the water to a boil over medium-high heat. Boil the rutabaga for 40 minutes; rutabagas take a long time to bring out their natural sweetness. Add the potatoes to the pot and cook until the potatoes are fork tender, about 15 minutes.
Heat the cream in a small saucepan over medium heat.
Drain the rutabagas and potatoes from the water and pass through a food mill. Add half the hot cream and cold butter into the milled rutabaga/potato mixture and stir vigorously to combine. Repeat this with the remaining cream and butter. Taste to make sure the seasoning is correct.