This is always a welcome addition at any barbecue.
In a medium bowl, combine mayonnaise, vinegar, and sugar; season with salt and pepper. Add cabbage, carrots, and scallions; mix to combine. Let stand at least 15 minutes at room temperature to soften cabbage, or cover and refrigerate up to 1 day.
If you don't have a julienne peeler, simply slice the carrots into ribbons with a vegetable peeler, then cut the ribbons into long, thin strips.