Tart and vibrant, sumac is a common seasoning in Middle Eastern cooking. We use it here to balance the sweetness of tomatoes, mixing it into an oniony vinaigrette. The tomatoes, along with an abundance of herbs, get gently tossed in the vinaigrette while a final spoonful of sumac adds a finishing sprinkle of flavor. Note that this salad is best made with perfectly ripe, in-season tomatoes. For added color and flavor contrast, add diced or coarsely crumbled feta cheese.