Preparation Combine vegetables and onion in a medium bowl; season generously with salt. Let sit until they start to release their water, 8–10 minutes, then begin to massage, gently at first so they don’t break, then harder as they begin to expel their liquid, until softened to about the texture of coleslaw. Pour off all excess liquid and add vinegar and oil; toss to coat. Taste and season with more salt if needed. Do Ahead: Slaw can be made 3 days ahead. Cover and chill.