Put the squash, onion, and potatoes in a large pot. Add the water. Bring to a boil, lower the heat, and cover. Simmer gently, stirring occasionally, for about 30 minutes.

Remove the vegetables with a slotted spoon, and purée in a food mill or food processor, using the cooking water to thin it out to the consistency desired. (If you’re preparing soup ahead of time, use all the cooking water, since the soup will thicken as it stands.)

Just before serving, add to the heated soup the butter, and season with salt and pepper. Cook 2 minutes more, and serve.

Note: Just before serving, you can add fresh parsley for color, or ¼ teaspoon grated nutmeg, or both. A swirl of crème fraiche or sour cream would be nice, too.