Toast the walnuts. In a small skillet on medium-low heat, swirl and toss the walnuts until they get soft, golden, and fragrant, about 7 minutes. Transfer to a cutting board to cool, then finely chop.
Prep the cucumber. Thinly slice ¼ of the cucumber and set aside for serving. Slice the remaining cucumber in quarters lengthwise, then use a metal spoon to scrape out and discard the seeds. Finely chop the remaining cucumber.
Make and chill the soup. In a large bowl, whisk together yogurt and ice water. Stir in the chopped cucumbers, herbs, raisins, and chopped walnuts. Season with a pinch of salt. Cover and chill for one hour, or up to 3 days.
Serve. Divide the chilled cucumber soup into four serving bowls. Sprinkle sumac and herbs on top and finish with a few cucumber slices to each bowl.
