Watermelon Salad with Feta and Cucumber Pickles

Sumac, which is sold in spice shops, gives it a subtle pine, sharp note, but don't worry if you can't find it: the taste of the salad will be very difficult without it. You can also substitute 1 teaspoon of ready-made horseradish for wasabi powder.
Combine the first 7 ingredients in a medium saucepan over high heat; bring to a boil. Remove the pan from the heat; allow to cool completely. Place the cucumber slices in a large bowl; pour the vinegar mixture over the cucumbers. Cover and refrigerate the cucumber and vinegar mixture for 8 hours or overnight.Combine the oil, vinegar, and sumac, if desired, in a small bowl, stirring well with a whisk. Strain the cucumbers; remove the liquid and sprigs of tarragon. Place about 2 tablespoons of cucumbers on each of 8 plates. Place 1 cup watermelon cubes on each plate. Sprinkle the watermelon evenly with the cheese, mint, olives, and dill. Drizzle the vinaigrette sauce evenly.