To prepare the dressing, combine the chopped peach, vinegar, oil, water, honey (if using), and salt in a blender, and blend until completely smooth. If the peach you're using is very sweet, you may not need the honey at all.
Rip the kale into small pieces, and then use your hands to massage the dressing into the leaves until they are tender and slightly wilted. Toss in the red onion and chopped peach, and allow the salad to marinate for 30 minutes before serving. Save any leftover salad in a sealed container in the fridge for up to 3 days.