1. Remove the flesh of the watermelon and cut it into small pieces. Remove and discard the seeds, then set aside.

2. Peel and slice the onion into rounds. In a small bowl, mix the vinegar, salt, and pepper, then whisk until the salt is dissolved. Slowly pour in the olive oil, a few drops at a time.

3. Add the chopped mint, taste and adjust the seasoning. In a large bowl, combine the watermelon, onion, and feta.

4. Pour the vinaigrette over the watermelon mixture and mix gently, until everything is evenly coated.

5. Garnish with sprigs of mint. To serve, divide the salad among individual plates and garnish with mint leaves.