Toast the walnuts in a pan over medium heat until they are fragrant and golden brown.
Then set them aside to cool.
In a bowl, mix mayonnaise, Greek yogurt, and lemon juice.
Add salt and pepper to taste as you stir.
Slice the apples into thin batons.
Toss them with a bit of lemon juice to prevent browning.
Chop the celery into small pieces and halve the grapes if they are large.
Combine apples, grapes, and celery in a large bowl.
Pour the dressing over the fruit and vegetable mix.
Add walnuts, then gently toss the salad to coat everything evenly.