How to Make It
Puree the cucumbers, avocado, yogurt, tarragon, lemon zest and juice, 1½ cups water, ½ teaspoon salt, and ¼ teaspoon pepper in a blender until smooth. Adjust the consistency with more water if desired. Chill if desired.
Spread the toast with the butter and top with the prosciutto, dividing evenly.
Serve the soup sprinkled with tarragon, accompanied by the prosciutto toasts.