Shave the cabbage with a sharp knife or mandoline so you have thin ribbons. Cut the scallions long and on the bias so you have pieces similar in shape to the cabbage. Toss the cabbage, scallions and cilantro in a large salad bowl.
Make the dressing by combining the sour cream, mayonnaise, sugar and the zest of the limes in a medium bowl. Season with salt and pepper and finish with a squeeze of lime juice. Pour the dressing over the cabbage mixture and toss to combine.