1.5 lb zucchini peeled halved lengthwise (3 to 4 medium)
6 servings zucchini thinly sliced
Equipment
bowl
sauce pan
blender
Directions
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.
Purée 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.
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.