Drain the canned chicken thoroughly and place in a large bowl. Use a fork to break apart any large chunks, maintaining some texture.
Add the diced celery, red onion, and apple to the bowl.
In a separate bowl, whisk together mayonnaise (or Greek yogurt), lemon juice, and Dijon mustard until smooth.
Pour the dressing over the chicken mixture and gently fold together until evenly coated.
Season with salt and pepper to taste. Add gradually since canned chicken already contains some sodium.
Fold in any optional add-ins like dried cranberries or walnuts.
Cover and refrigerate for at least 30 minutes before serving to let flavors meld.