Prepare the yogurt dressing by combining the yogurt, apple cider vinegar, mustard, honey, salt, dried parsley, crushed red pepper flakes, and pepper in a bowl. Whisk until well-combined.

Combine chickpeas, celery, apple, grapes, onion, and walnuts in a large bowl. Stir in the dressing and toss until evenly coated. Refrigerate for about 30 minutes before serving, or up to 5 days.

Serve salad over fresh spinach.