summer squash soup

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 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