Rough up the green onions slices a little with your fingers so all the little layers of the scallion whites separate. Toss the cabbage, red onions, and green onions together in a large bowl until everything is thoroughly mixed. You can make the slaw up to this point up to a day in advance as long as you keep it refrigerated. Before serving, simply bring the slaw back to room temperature, make the dressing, and toss.
Stir the remaining ingredients together in a small bowl until blended, then pour over the vegetables. Serve within 1 hour of dressing or the cabbage will get wilty and sad.