In a large mixing bowl, whisk together the mayonnaise, lemon juice, salt, and black pepper until the dressing is smooth.
Incorporate the diced apples, sliced celery, and halved grapes into the bowl.
Toss the fruit and vegetables with the dressing until all components are evenly and thoroughly coated.
Gently fold in the toasted chopped walnuts to maintain their texture.
Place individual lettuce leaves on serving plates or a chilled platter.
Mound the salad mixture onto the lettuce leaves and serve immediately or refrigerate for up to one hour.