1 1/2 cups chopped, peeled potatoes, such as Yukon Gold
1/2 cup chopped cauliflower
1/2 cup celery
1 large handful fresh dill springs, including stems. Plus 1/4 cup chopped dill leaves
1 handful whole fresh chives
1 cup non-fat plain yogurt
1 tablespoon butter
1 1/4 cups finely chopped peeled cucumber, plus half-moon slices for garnish
freshly ground pepper to taste
Combine broth, potatoes, cauliflower and celery in a large saucepan. Lay the dill and chives on top. Bring to boil over medium heat. Boil briskly until the vegetables are very tender when pierced with a fork (15 to 18 minutes). Remove from heat and allow to cool slightly.
Combine 1/4 cup each of chopped dill and chives with yogurt in a food processor. Process until herbs are finely chopped and mixture is slightly green. Transfer half of the mixture to a bowl: cover and refrigerate to use as a garnish.
Using a fork, lift and discard the herbs from the saucepan. Transfer the vegetables to the processor with a slotted spoon, leaving he broth in the pan. Add butter and process until smooth, scraping the sides as needed.
Add diced cucumber to broth. Bring to a gentle boil and cook until the cucumber is just cooked through but still slightly crisp (about 3 minutes). Remove from heat. Thoroughly stir pureed vegetables into cucumber mixture. Season with pepper. Refrigerate until chilled at least 3 hours.
Serve soup with a dollop of reserved, herbed yogurt. Garnish with sprigs of dill, chopped chives and/or cucumber halves.