Preheat the oven to 350ºF.
If using regular yogurt instead of Greek-style yogurt, place in a strainer lined with a paper towel. Put the strainer over a bowl and place in the refrigerator to drain and thicken for 20 minutes.
Spread the walnuts on a baking sheet and toast in the oven for 8-9 minutes. Set aside to cool; then chop coarsely.
In a small bowl, combine the thickened or Greek-style yogurt, mayonnaise, lemon juice, and honey until smooth.
In medium sized bowl, toss together the toasted walnuts, celery, and apples. Pour the dressing over the salad and toss to combine.