Sumac--available at spice stores--adds a subtle piney, tangy note, but don't worry if you can't find it: The salad will still have loads of flavor complexity without it. You could also sub 1 teaspoon of prepared horseradish for the wasabi powder.
1 cup sugar
1 cup cider vinegar
1/2 cup water
1 teaspoon kosher salt
2 teaspoons mustard seeds
1 1/2 teaspoons wasabi powder
3 tarragon sprigs
1 English cucumber, thinly sliced
3 tablespoons extra-virgin olive oil
2 tablespoons sherry or white wine vinegar
1/2 teaspoon ground sumac (optional)
8 cups cubed red watermelon
3 ounces feta cheese, crumbled (about 3/4 cup)
1/2 cup chopped fresh mint
1/4 cup sliced pitted kalamata olives
1/4 cup chopped fresh dill
How to Make It
Combine first 7 ingredients in a medium saucepan over high heat; bring to a boil. Remove pan from heat; cool completely. Place cucumber slices in a large bowl; pour vinegar mixture over cucumbers. Cover and chill cucumber and vinegar mixture 8 hours or overnight.
Combine oil, vinegar, and sumac, if desired, in a small bowl, stirring well with a whisk. Strain cucumbers; discard liquid and tarragon sprigs. Place about 2 tablespoons cucumbers on each of 8 plates. Place 1 cup watermelon cubes on each plate. Top watermelon evenly with cheese, mint, olives, and dill. Drizzle evenly with vinaigrette.