1⁄3 , seeded and cut in 4 rectangles (2 X 1 1/2 Inches Each, For Garnish)
Fresh dill sprigs
4 (For Garnish)
In a 3-quart saucepan, melt the butter over medium heat, add the leek, onion, and cucumber, and saute for 2 minutes.
Add the stock and bring to a boil.
Reduce the heat and simmer the vegetables, uncovered, for 20 minutes.
Puree the soup in a food processor or blender in several batches (the hot liquid will increase in volume dramatically as it is pureed).
Pour into a bowl, let cool to room temperature, and refrigerate until cold, about 1 1/2 hours.
The soup can be prepared ahead to this point.