Using a julienne peeler, mandolin or grater, cut the kohlrabi, carrot, and apple into long, thin matchsticks. Transfer to a large mixing bowl.
Finely chop the spring onions and add them to the bowl.
In a small bowl or jar, whisk together the olive oil, Dijon mustard, apple cider vinegar, and honey (or maple syrup).
Pour the dressing over the slaw and toss until everything is evenly coated. Season with salt and black pepper to taste.
Sprinkle in the chopped parsley and mix again. Let the slaw sit for at least 10 minutes before serving to allow the flavours to blend.