Ingredients

• 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 lb), peeled, halved,
• 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

Instructions

1. 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.
2. Working in batches, purée; soup in a blender until smooth (use caution when blending hot liquids) and transfer to a bowl. Return puré to cleaned pot and thin with water if desired; simmer 3 minutes. Season with salt.
3. Make pistou 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 pistou into each bowl of soup.
4. 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.
5. Swirl 1 tablespoon pistou into each bowl of soup.