Sprinkle the cavities of the squash halves with salt and arrange the squash, inverted, in one layer in a buttered baking dish. Bake the squash, covered with foil, in the middle of a preheated 375F. oven for 1 hour and let it cool until it can be handled. Scoop out the squash pulp, discarding the skin, and in a food processor puree it with the butter. Transfer the puree to a saucepan and simmer it, stirring occasionally, until the excess liquid is evaporated. Season the puree with the nutmeg and salt and pepper. The pure3 may be made 1 day in advance and kept covered and chilled.