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
Garnish: thinly sliced or torn zucchini blossoms
Cook shallots in oil in a 4-quart heavy saucepan over moderate heat, stirring occasionally, until softened, about 5 minutes.
Add zucchini, zest, salt, and pepper and cook, stirring occasionally, until zucchini is softened, about 5 minutes.
Add broth and water and simmer until zucchini is tender, about 3 minutes.
Pur�e zucchini mixture, including zest, along with parsley and dill in a
blender (in 2 batches if necessary) until smooth (use caution when blending hot liquids).
Transfer to a metal bowl, then set bowl into a larger bowl of ice and cold water (to quick-chill).
Cool, stirring occasionally, about 20 minutes.
Stir in buttermilk and season with salt.
Cooks' note:
Soup (with buttermilk or yogurt) can be made 1 day ahead and chilled, covered.
(If making soup ahead, cooling in ice bath is not necessary.)
Stir well before serving.
Makes 6 servings.