Preparation Melt butter in heavy medium saucepan over medium heat. Add leeks; sauté just until soft but not yet brown, about 5 minutes. Add broth and potato. Simmer until potato is very tender, about 12 minutes. Using slotted spoon, transfer potato and leeks to blender. Add half of cooking liquid and blend to coarse puree. Add cream and blend, using on/off turns. Transfer to large bowl. Combine cucumber, chives and remaining cooking liquid in blender; puree until smooth. Mix into leek puree. Season with salt and pepper. Chill, at least 4 hours and up to 6 hours. Stir soup and ladle into bowls. Garnish with radishes, if desired.
Preparation Melt butter in heavy medium saucepan over medium heat. Add leeks; sauté just until soft but not yet brown, about 5 minutes. Add broth and potato. Simmer until potato is very tender, about 12 minutes. Using slotted spoon, transfer potato and leeks to blender. Add half of cooking liquid and blend to coarse puree. Add cream and blend, using on/off turns. Transfer to large bowl. Combine cucumber, chives and remaining cooking liquid in blender; puree until smooth. Mix into leek puree. Season with salt and pepper. Chill, at least 4 hours and up to 6 hours. Stir soup and ladle into bowls. Garnish with radishes, if desired.