SUMMER-SQUASH SOUP WITH PARSLEY MINT PISTOU

Steps:

• Make soup:
• 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, 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.
• 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.