Wash and core the apples, then dice them into small pieces.
Halve the grapes and set them aside.
Chop the celery into small, bite-sized pieces.
In a large mixing bowl, combine the diced apples, grapes, and chopped celery.
In a separate bowl, mix mayonnaise and lemon juice until smooth.
Pour the dressing over the fruit and celery mixture.
Gently fold the ingredients together to coat evenly.
Add chopped walnuts, seasoning with salt and pepper to taste.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes.
Serve chilled on a bed of lettuce leaves.