In a medium bowl, whisk together yogurt and lemon juice; season with coarse salt and ground pepper.
Add apple, celery, red seedless grapes, and toasted walnuts; toss to coat with dressing. Cover; refrigerate until ready to serve, up to overnight.
Divide salad in half; serve each portion on 2 Boston lettuce leaves.
Had a friend come over to visit and she let me know she was a vegetarian. Looked up this recipe and I had some apples, non fat- yogurt, lemon, celery, walnuts but, no grapes. Instead of the grapes, I used dried cranberries. I also, added some local honey to the recipe for a sweeter version, according to her, it was YUMMY and I have to say, I will be making more of this Yogurt Waldorf Salad from now on!!! I added some chicken to mine for those who are not vegetarian
As someone who absolutely despises mayonnaise, I am thrilled to find a waldorf salad that eliminates it completely! I added a few drizzles of honey at the end to make it a little sweeter.