1. Place the celery in a bowl and cover with cold water. Refrigerate. In a small skillet over medium heat, toast the nuts for 3 to 4 minutes, shaking the pan for even browning. Transfer to a plate to cool.
2. In a salad bowl, combine lemon juice, mustard, garlic, honey, and salt. Whisk in the oil, and season with pepper. Peel and quarter the apple; core the quarters, and slice each quarter into 2 wedges. Cut the wedges across into thin slices, add them to the dressing, and toss. Drain the chilled celery and blot dry with a paper towel. Add the celery, celery leaves, and walnuts to the apples and toss. Serve immediately.