Deborah Madison describes this soup as being "like putting on the first sweater of the season: it just feels so good."
Preheat the oven to 425 degrees F. Cut the squash in half, scrape out the seeds [reserve seeds for step 2], then cut each half into thirds. Put the pieces in a large baking dish or roasting pan with the pears and all but a few slices of ginger. [tuck ginger slices under larger pieces of squash or pears, so ginger does not burn]. Brush with oil, season with salt, and bake until fragrant and tender, about 1 hour. Turn the pieces once or twice so that they have a chance to caramelize on more than one surface. If the squash seems very dry (some varieties are), add 1 cup water to the pan to create steam and cover with foil. When the squash is tender, transfer everything from the pan to a cutting board, add 1 cup of water to the pan, and scrape to dissolve the juices, reserving the liquid. [don't do this if you used aluminum foil on a baking dish, or you will get a metallic taste in the liquid - so just skip this step]. Scrape the flesh of the squash away from the skins. [reserve the skins for next step]. You should have about 2 cups.
To make a stock, bring 6 cups water to a boil and add the seeds and, eventually the squash skins, the remaining ginger, and ½ teaspoon salt. Lower the heat and simmer, covered for 20 to 25 minutes. Madison notes that "you do not need to make this stock, but the possibilities the roasted skins and squash seeds hold for extra flavor make it worth simmering them." [I simmered the skins but skipped the seeds].
Meanwhile, melt the butter in a soup pot. Add the onion, give it a stir, and cook over medium-low heat, stirring frequently, until it begins to brown a bit and is fragrant, about 10 minutes. Add the pears, ginger, and squash, then the reserved deglazing water. strain the stock into the pot. Bring to a boil, then lower the heat and simmer, covered for 25 minutes. Cool briefly, then puree until smooth and pass through a food mill or strainer to ensure a silky texture. Serve as is or swirl in the creme fraiche [or yogurt, like me].
* Dice a pear or an apple, sprinkle with a little sugar, and caramelize in a little butter or oil. Use these "croutons" as a finishing touch.
* Omit the ginger and add cooked wild rice to the finished soup.
* Crisp thin strips of fresh ginger in oil or butter and add a cluster to each servingd
Madison's wine pairing: For wine, turn to a heartier white wine with bright tropical fruit, such as Sanford's Chardonnay from Santa Barbara County.