1 small seedless watermelon, peeled and cut into 1/2-inch cubes
1 small honeydew melon, peeled and cut into 1/2-inch cubes, optional
1 (8-ounce) block feta cheese, cut into 1/2-inch cubes or coarsely crumbled
1 tablespoon honey, such as buckwheat
1/4 cup fresh mint leaves
1 lime, juiced
1/2 teaspoon sumac, for sprinkling, see Cook's Note*
*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.