Buttermilk and avocado lend tang and creaminess to this light, bright summer soup. To read more about cucumbers, visit Wall Street Journal's "Off Duty" section.
Peel, halve and, if necessary, seed the cucumbers. Dice enough cucumbers to set aside 1 cup for garnish; roughly chop the remaining and place in a blender.
Add the buttermilk, avocado, herbs, vinegar, sugar, and salt to the blender and blend until very smooth, about 2 minutes. Strain the soup through a fine mesh sieve and thin with water to desired consistency.
Season soup with salt and pepper, cover, and chill at least 2 hours.
Divide soup among bowls, drizzle with oil, garnish with croutons, and serve.