Core and dice or thinly slice the apples; toss immediately with lemon juice to prevent browning.
Wash and halve the grapes; pat dry.
Trim and thinly slice celery on a diagonal for better texture.
Lightly toast walnuts in a dry skillet over medium heat for 2–3 minutes until fragrant; cool and chop.
In a bowl, whisk together mayonnaise, yogurt (or sour cream), honey, salt and pepper until smooth.
Taste and adjust acidity with a little extra lemon or sweetness with honey.
Combine apples, grapes, celery and most walnuts in a large mixing bowl.
Pour the dressing over the fruit and vegetables and toss gently to coat evenly.
Reserve a few walnut pieces and a few apple slices for garnish; chill the salad 10–15 minutes if desired.
Before serving, toss again and sprinkle reserved walnuts and apple slices on top.