summer squash soup

Name: summer squash soup
Cuisine: American
Ingredients: For squash soup; 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; For pistou**; 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