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 hours. Rinse cabbage mixture thoroughly in cold water and spin it dry in a salad spinner.
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.
Quarter, core, and shred the apples. Toss the cabbage mixture with the apples, scallions, and about 1⁄2 cup of the dressing. Refrigerate until chilled and the flavors 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.