Add walnuts to a skillet over medium heat and toast for 2-5 minutes or until fragrant, tossing occasionally. Remove from pan and let cool.
In a large bowl combine diced apple, grapes, celery, walnuts, yogurt, lemon juice as well as raisins and parsley (if using). Taste and season with salt and add pepper if desired.
Serve salad over a bed of lettuce, if desired.
Notes
To store: Store leftovers (without the bed of greens) in an airtight container in your refrigerator for 3-4 days. If the salad seems a little dry you can add a little more yogurt or mayo before serving.