This is a really simple soup with only a few ingredients, that even though it takes a long time to simmer on the stove, it is pretty simple to make. Make a big batch and freeze leftovers!
- 8 tbsp. unsalted butter, divided
- 2 large sweet onions (14 oz.), finely sliced (5½ cups)
- 3 large celery stalks (6 oz.), diced (1½ cups)
- 1 large potato (10 oz.), diced or finely sliced (2 cups)
- 2 lb. pumpkin, Hubbard, or other firm, red or yellow-fleshed squash, peeled, seeded, and diced (6 cups)
- Kosher salt
- 1⁄4 cup flat-leaf parsley, finely chopped
- Freshly ground black pepper
In a heavy saucepan, melt 4 tablespoons butter over low heat. When the butter is hot, add the onions and celery, and cook, covered, stirring occasionally with a wooden spoon, until the onions are soft and translucent but not colored, 3–4 minutes. Add the potato, squash, and salt and continue to cook over low heat, stirring regularly, until the squash begins to disintegrate, 25–30 minutes. Add 8 cups of boiling water to the vegetable mixture, partially cover, and continue cooking until the vegetables are very soft, about 30 minutes.
Set a food mill over a large bowl, mill the soup, then return it to the pot and set over low heat to rewarm slightly. Add the parsley and freshly ground black pepper, stir, and remove from heat. Cut the remaining butter into small pieces and stir into the soup before serving.