How to Make It
In a blender, puree the cucumbers, a handful at a time, until smooth. Add the watercress, 6 of the dill sprigs, the vinegar, 1 tablespoon of the oil and 1 teaspoon of salt and puree until smooth.
Transfer the soup to a bowl and stir in the yogurt. Refrigerate until chilled, about 2 hours. Season with salt and pepper and pour into bowls. Garnish with the radishes, scallions and the remaining 4 dill sprigs. Drizzle with a little olive oil and serve.