1) For the salad: Quarter and core the cabbage. Set up a food processor with the slicing blade. Slice the cabbage and then transfer it to a colander. Switch to the grating blade and shred the carrots; add to cabbage. Toss the cabbage and carrots with the salt and set colander in the sink until the vegetables wilt, 1 to 4 hrs. Rinse cabbage mixture thoroughly in cold water and spin it dry in a salad spinner.
2) Meanwhile, make the dressing: Whisk the vinegar with the honey, mustard, celery seeds, salt, and black pepper to taste in a small bowl. Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream to make a smooth, slightly thick dressing.
3) Quarter, core, and shred the apples. Toss the cabbage mixture with the apples, spring onions, and about 120ml of the dressing. Refrigerate until chilled and the flavours come together, about 1 hour. When ready to serve, toss the watercress with the remaining dressing. Make a ring on a platter with the watercress, and mound the coleslaw in the middle of the ring.
Know-How: Salting the cabbage gives the slaw its characteristic tender bite, as well as expressing excess liquid. If you skip this step, your salad will be soggy and bland.