Toss the coleslaw mix together with the scallions and red onion in a large bowl until everthing is well mixed. You can make the slaw up to this point up to a day in advance as long as you keep it refrigerated.
When ready to serve, bring the slaw back to room temperature. Stir the dressing ingredients together in a small bowl until blended or shake together in a sealable plastic container.
Taste for salt and add a couple pinches if needed. Pour over the slaw, toss well, and serve within 1 hour or it will get wilted.