Combine the cabbage, carrots and onions in a large serving bowl. Douse the vegetables with the malt vinegar, season with salt and let sit for at least 1 hour. This will soften the cabbage and make it seem almost cooked but still with a great texture.
Stir in the mayonnaise and the mustard. Taste for seasoning. The final outcome should be a creamy and tart slaw. Serve cold or at room temperature. Refrigerate leftovers.