Free the cabbage of the outer leaves, cut out the stalk and grate the cabbage finely. Peel and grate the carrot. Peel the onion and cut into thin strips.
For the dressing, combine the mayonnaise with the sugar, vinegar and mustard and season with salt and pepper.
Mix the dressing into the cabbage, carrot and onion. Cover and leave to marinate for at least 4 hours, preferably overnight.
Season to taste before serving.