In a jar with a lid, combine the olive oil, vinegar, mustard and salt and pepper to taste. Shake vigorously and set aside.
In a large bowl, combine the shredded Napa, red and green cabbages with the parsley and celery seeds. Shake the dressing again and pour over the cabbage; toss to coat.
Trim the ends of the reserved cabbage leaves and place a large spoonful of marinated slaw in the center of each leaf; refrigerate until ready to serve.
