Thinly slice the cabbage and apple (if using) into thin strips. Use a mandoline slicer (this is my favorite) for precise and thin cuts. A sharp knife will work as well. Using a julienne peeler or a mandoline slicer, cut the carrots into strips (you can also use a coarse grater if you don’t have a julienne cutter). The thinner your vegetables, the better they'll absorb the dressing. Roughly chop the parsley.
In a large bowl, combine the sliced cabbage, carrots, parsley, and apple. Toss until they are well mixed.
In a separate small bowl or jar, combine the extra virgin olive oil, apple cider vinegar, Dijon mustard, maple syrup, salt, and pepper. Whisk or shake the jar until the dressing is thoroughly combined. Taste and adjust the seasoning if needed.
Pour the dressing over the mixed vegetables and toss until everything is evenly coated.
Stir in the chopped parsley.
Let the salad sit for at least 10-15 minutes before serving to allow the flavors to melt together. This salad can be made ahead and kept in the fridge for a couple of days. The flavors will intensify over time.