- 3/4 stick (6 tablespoons) unsalted butter, cut into pieces
- 1 medium onion, halved lengthwise and thinly sliced crosswise
- 1/2 teaspoon salt
- 2 lb. yellow summer squash, halved and thinly sliced
- 2 carrots, thinly sliced
- 1 yellow-fleshed potato (1/2 pound), peeled, halved, and thinly sliced
- 4 cups chicken stock or reduced-sodium chicken broth
- 3/4 cup loosely packed fresh mint leaves
- 1/2 cup loosely packed fresh flat-leaf parsley sprigs
- 1 large scallion, chopped (1/2 cup)
- 1/4 cup extra-virgin olive oil
- 2 tablespoons water
- 1/4 teaspoon salt
- Melt butter in a 6- to 8-quart wide heavy pot over moderate heat, then cook onion with salt, stirring, until softened, about 8 minutes.
- Add squash, carrots, potato, and stock and bring to a boil. Reduce heat, then simmer, partially covered, until vegetables are very tender, about 20 minutes.
- Remove from heat and cool soup, uncovered, 10 minutes.
- Working in batches, puree; soup in a blender until smooth (use caution when blending hot liquids) and transfer to a bowl.
- Return purée to cleaned pot and thin with water if desired; simmer 3 minutes.
- Season with salt.
- While vegetables simmer: Pulse mint, parsley, and scallion in a food processor until finely chopped.
- With motor running, add oil in a stream, then add water and salt, blending until incorporated.
- Swirl 1 tablespoon pesto into each bowl of soup.