Halve watermelon; scoop out balls of melon with a melon baller into a bowl. Separate leaves of lettuce and arrange like cups in a layer on a serving platter. Place melon balls on the lettuce. Crumble feta cheese over the salad and top with toasted cashews.
Whisk rice vinegar, olive oil, salt and pepper together. Drizzle over the salad. Garnish with mint or basil leaves.