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