Finely slice the red cabbage as thinly as you can, then grate the carrot on the fine side of a box grater.
Thinly slice the apple just before using, then roughly chop the slices into irregular pieces; very finely slice the red onion.
Toast the walnuts in a small dry pan over medium heat for 2–3 minutes, shaking often, until fragrant and lightly coloured; tip out to cool, then roughly crumble.
Add the cabbage, carrot, apple, and onion to a large bowl and toss to loosen and distribute evenly.
Whisk the olive oil, cider vinegar, Dijon, honey/maple, salt, and pepper until emulsified, then stir in the Greek yogurt until smooth and pourable.
Pour the dressing over the vegetables and toss until lightly coated; it should look glossy, not wet or pooled at the bottom.
Leave for 10 minutes to soften the cabbage slightly while keeping its crunch.
Fold through the toasted walnuts (and herbs, if using) just before serving.