This is a mashup, of sorts, of the Greek classic called karpouzi salata (watermelon salad) and Palestinian salatat jarjeer (arugula salad). Refreshing, summery and colorful, this dish juxtaposes savory, sweet and bitter flavors with crisp, creamy and crunchy textures. Feta is the cheese typically used in Greek watermelon salad, but here we opted for the milder flavor and richer texture of fresh goat cheese. Ground sumac is a deep-red powder made by grinding the dried berries of the sumac tree. Used extensively in Middle Eastern cooking, sumac has a tart, bright, citrusy flavor. Look for it in either the international section or the spice aisle of the supermarket, or in Middle Eastern grocery stores.