Start by chopping your cooked chicken into bite-sized pieces if it isn't shredded already. Dice the celery and apple into small, even chunks, and halve your grapes. Having everything prepped and ready makes the rest of this recipe come together in minutes.
In a large bowl, whisk together the Greek yogurt, mayonnaise, Dijon mustard, honey, salt, and pepper until it's completely smooth. Taste it here and adjust the seasoning if it needs a touch more salt or honey. This dressing is really the backbone of the whole salad.
Add the chopped chicken, apple, celery, grapes, and walnuts directly into the bowl with your dressing. Using a spatula, gently fold everything together until each piece is coated evenly. Try not to overmix, since you want the apple and walnuts to stay crisp.
Cover the bowl and refrigerate it for at least 30 minutes before serving, though it tastes even better after an hour or two. Chilling lets the flavors settle together and gives the salad a refreshing, cool bite that makes it so satisfying on a warm day.