Peel rutabagas, and cut them in large chunks. Put them in a pan of cold salted water, cover, and bring to a boil. Simmer until very tender, 30 to 40 minutes. Drain the rutabagas, return them to the pan, and heat gently for 2 or 3 minutes to dry them. Mash the chunks with a potato masher or fork, the puree will always be slightly fibrous. Work in butter with a generous grate of nutmeg, taste, and adjust the seasoning.