1 fresh lemongrass stalk, root end trimmed and 1 or 2 outer layers discarded
1 large onion, chopped
butter or oil
1 1/2- 2lbs (4 C) butternut or other sweet squash, peeled, seeded, and cut into 1-inch pieces
2 1/2 cups fresh or frozen corn kernels
5 cups water
Slice bottom 5 inches of lemongrass into chunks. Melt a bit of butter or oil in a pot and combine lemongrass, onion, and 1 teaspoon salt. Cook until onion is softened, about 10 minutes. Add squash, corn, water, 1 1/2 teaspoons salt, and 1/2 teaspoon pepper and bring to a boil over high heat.
Reduce heat and simmer, covered, stirring occasionally, until squash is tender, about 10 minutes. Remove from heat and discard lemongrass (if you can find it).
Purée soup and then strain it through a fine-mesh sieve, pressing hard on and then discarding solids. Season with salt and pepper.