To prepare this watermelon feta salad recipe start by removing the rind and seeds from the watermelon, and cut into triangular chunks. Cut the feta cheese into small cubes, or bigger if you prefer, and place both into a large bowl. Slice the onions into paper-thin slices and place in the bowl with the rest of the ingredients, add the chopped mint leaves and the olives and set aside. (If you choose to prepare this watermelon feta salad recipe using cucumber too, then peel and slice the cucumber into half-moons and place in the bowl with rest of the ingredients). Prepare the dressing for the watermelon feta salad. In a small bowl add the ingredients. Whisk the ingredients to combine, taste and adjust the seasoning. Pour the dressing over the watermelon feta salad and toss to coat. Divide the watermelon feta salad among individual serving plates and serve immediately.