Slice apple, celery and cabbage leaves thinly, on the diagonal.
Toss with parsley, oil, orange and lemon juice, salt, pepper, mustard and maple syrup in a large bowl.
Using a knife, cut away the peel and pith of grapefruit, then section into supremes. Set aside a few nice supremes for garnish and cut the others in half. Squeeze grapefruit to get all remaining juice into the salad.
Add arugula and toss. Garnish with reserved supremes.