Cut squash in half, lengthwise, and clear cavity of seeds. Peel. Chop squash flesh into cubes. Steam until just tender, about 20 minutes. Drain, reserving some of the cooking liquid. Puree squash with electric hand beater or food processor, adding liquid until desired consistency. Add butter if desired.
Heat oil in heavy skillet. Add onion and sauté until translucent. Add to squash and stir until blended. Add salt and pepper to taste. Serve topped with minced Italian parsley, nutmeg, and cinnamon.