Cut the flesh from the melon and cut into bite size pieces, removing and discarding the seeds, and set aside. Peel and slice the onion into rings.
In a small bowl, combine the vinegar, salt, pepper, and whisk until salt is dissolved. Slowly whisk in the olive oil, a few drops at a time. Add in the chopped mint, taste, and adjust seasonings.
In a large bowl, combine the melon, onion, and feta. Pour the dressing over the melon mixture and toss gently until everything is coated and evenly mixed. Garnish with mint sprigs.
To serve, divide salad among individual plates and garnish with mint leaves.