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.
Clio, Boston, Mass.