Step 1 Heat the oven to 400 degrees. Line a jellyroll pan (a baking sheet with sides) with aluminum foil. If the squash is whole, cut it in half and scoop out the seeds with a large spoon.
Step 2 Place the squash cut-side-down on the jellyroll pan and add about one-fourth inch of water. Roast 40 minutes, then turn cut-side-up and cook until the squash begins to collapse and is soft enough to be easily pierced with a knife, about 20 more minutes.
Step 3Remove the squash from the oven and cool briefly, about 5 minutes. Peel, core and chop the apple. When the squash is just cool enough to handle with a kitchen towel, use a spoon to scoop the flesh into a food processor or blender (a food processor will make a fairly dense, sticky puree; the blender will be lighter and smoother).
Step 4Add the apples and the whipping cream and puree, stopping and scraping down the sides as necessary. Add the ginger and the butter and continue pureeing until you can't see any traces of the butter. Add one-fourth teaspoon salt, pulse a couple of times to mix well, then taste and add more salt as necessary.
Step 5Scrape the puree into a small saucepan and warm, covered, over medium heat, 2 minutes. The recipe can be prepared 30 minutes in advance and kept warm in a covered pan over very low heat. Serve hot.