In a large mixing bowl, whisk together the olive oil (or hazelnut oil), apple cider vinegar, honey, whole grain mustard, and sea salt until the ingredients are fully combined and the dressing is smooth.
To the bowl, add the apple matchsticks, kohlrabi matchsticks, red onion, toasted hazelnuts, and chopped parsley. Toss all ingredients thoroughly to ensure they are evenly coated with the dressing.
Season the slaw with freshly ground black pepper and additional salt, if necessary. Garnish with extra parsley before serving to enhance the dish’s visual appeal and freshness.