Combine apple and cabbage in a large bowl. Set aside.
Place vinegar, sugar, mustard and salt flakes in a small saucepan over high heat, and stir until sugar dissolves. Bring to the boil then pour over apple slaw. Toss to combine, then chill for 15 minutes to soften and lightly pickle.
Add lemon juice, parsley, walnuts, chives and oil to slaw, and toss to combine. Serve with grilled chicken.