Whisk the vinegar with the mustard, salt and pepper, to taste, in a large 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. Add the cabbage, scallions, and bell pepper and toss until the vegetables are coated. Serve now or cover and refrigerate for up to 4 hours.
Before serving, grate the apple (with the skin but not the seeds) directly into the slaw, add the watercress leaves, and toss again. Season, to taste, with salt and pepper.
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