Combine vegetables and onion in a medium bowl; season generously with salt and mix lightly. Let sit for at least 10 minutes until they start to release their water.

Begin to massage the vegetables, gently at first. Then, more firmly as they start to release liquid. Continue massaging until the vegetables are about the texture of coleslaw.

Pour off all excess liquid, add vinegar and oil, and toss to coat. Taste and season with salt if desired.