Cut the core from the cabbage quarters and then shred the remaining cabbage finely. Place into a colander. Coarsely grate or use a mandolin/cut carrots with a knife to cut fine julienne. Add the salt, sugar and vinegar then toss together using your hands, and leave in the sink to drain for an hour.
Press down the vegetables to squeeze out any excess liquid, then tip into a bowl and add the spring onion and toss through again with your hands.
In a bowl, mix the mayo with the wholegrain and Dijon mustard, then spoon all over the crunchy veg and mix through until well dressed.