Wash and dry the apples and celery.
Core and chop the apples into bite-sized pieces.
Slice the celery stalks into thin pieces.
In a large mixing bowl, combine the chopped apples and celery.
Add the halved grapes to the mixture.
Chop the walnuts and add them to the bowl.
In a separate bowl, mix mayonnaise and lemon juice.
Pour the dressing over the salad and toss gently.
Season with salt and pepper to taste.
Serve immediately on a bed of lettuce.