1. Puree the buttermilk, yogurt, avocado, and a quarter of the peeled cucumber in a blender until smooth, then pour it into a bowl. 2. Mash the garlic with 1/2 teaspoon salt and stir it into the puree along with the scallions, herbs, chile, and lime zest. Season to taste with salt, pepper, lime juice, and sweetener, if needed. Seed and finely dice the remaining cucumber and add to the soup. Cover and refrigerate until well chilled. Just before serving, taste and correct the seasonings. 3. Ladle the soup into bowls, then cover the surface with the chives, radishes, herb sprigs, and pistachios.