1/4 cup dried rose petals, crushed (optional, see Note)
2 cups 2 percent plain Greek yogurt
1 1/2 cups ice water
1/2 cup golden raisins
1/2 seedless cucumber, peeled and finely diced (1 cup)
1/4 cup finely chopped mint
1/4 cup finely chopped dill
1/4 cup finely chopped chives
Salt and freshly ground pepper
Ground sumac, for garnish (optional, see Note)
Preheat the oven to 350°. Spread the walnuts in a pie plate and toast for about 10 minutes. Let cool, then finely chop.
Meanwhile, in a small bowl, cover the rose petals with cold water and let stand until softened, about 20 minutes. Drain the petals and squeeze dry.
In a large bowl, whisk the yogurt with the ice water. Stir in the raisins, cucumber, mint, dill, chives, walnuts and rose petals and season with salt and pepper. Refrigerate until very cold, about 1 hour. Serve the soup in shallow bowls, sprinkled with sumac.