Chilled Zucchini Soup

Ingredients

• 1/4 lb shallots, thinly sliced crosswise (1 cup)
• 2 tablespoons extra-virgin olive oil
• 1 1/2 lb zucchini (3 to 4 medium), peeled and halved lengthwise, then cut crosswise into 1/8-inch-thick slices
• 2 (2- by 1 1/2-inch) strips fresh lemon zest
• 1 teaspoon salt
• 1/4 teaspoon black pepper
• 1 3/4 cups reduced-sodium chicken broth (14 fl oz)
• 1 3/4 cups water
• 1 cup loosely packed fresh flat-leaf parsley leaves
• 1 tablespoon finely chopped fresh dill
• 1/2 cup well-shaken buttermilk or plain yogurt