In a small bowl, use a fork to whisk together all the dressing ingredients except the sumac.
In a large bowl, and using clean hands, toss all the salad ingredients except the flatbread pieces. Add the dressing, 1 teaspoon of the sumac, and toss. Let the salad rest to meld the flavours.
Before serving, arrange the toasted flatbread croutons over the salad and sprinkle the other t