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