Cut the cabbage into wedges. Remove the tough core and discard and then finely shred the cabbage. Place in a large mixing bowl.

Peel the celeriac (this is best done with a small sharp knife) then coarsely grate the celeriac and the carrots and add to the bowl.

Peel, core and thinly slice the apples (if using). Toss the apples together with the prepared vegetables.

In a small bowl, combine the yogurt, mayonnaise, mustard, salt and pepper. Pour over the vegetables and toss together to mix well.

Chill until ready to serve.