Cut the flesh from the melon and cut into bite-size pieces, removing and discarding any seeds. Place watermelon pieces in a large serving bowl.
Peel and slice the onion into rings. Gently combine the onion rings and watermelon together.
In a small bowl, combine the vinegar, salt, and pepper; 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. Pour the dressing over the melon/onion mixture and toss gently until everything is coated and evenly mixed.
To serve, divide salad among individual plates and garnish with mint leaves.
Serves many.
Watermelon Salad with Mint Leaves Recipe: https://whatscookingamerica.net/salad/watermelonsalad.htm