Soak the sliced red onions in a small bowl of ice water for 10 minutes to remove their harsh bite, then drain them well.
Combine the cubed watermelon, sliced peaches, drained red onions, and chopped mint in a large, wide serving bowl.
Shake the olive oil, fresh lime juice, honey, salt, and black pepper in a small jar until fully emulsified.
Pour the dressing over the fruit mixture and toss very gently with two spoons to avoid bruising the peaches.
Sprinkle the crumbled feta cheese over the top of the salad immediately before serving to prevent it from dissolving.
