Assemble the watermelon and honeydew melon on a platter. Top with the feta cheese and drizzle with honey. Tear the mint leaves over the salad and sprinkle lime juice over the top. Sprinkle with the sumac. Serve.
*Sumac is a pretty pinkish-red powder whose tart, sour character is treasured in Middle Eastern cooking. It's made from the berry of a bush that grows wild in the region. You can find it at better supermarkets, Indian and Middle Eastern markets, and online.