Summer Squash Soup2018-01-30
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 pure to cleaned pot and thin with water if desired; simmer 3 minutes. Season with salt. 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.