In a large bowl, add chicken, grapes (or raisins), walnuts, apple, celery, gently mix
In a small bowl, mix together the mayonnaise, salt, pepper, and apple cider vinegar, and poppy seeds
Pour mayonnaise "dressing" over the chicken mixture, and stir, ensuring everything is evenly covered
Chill for 2 hours, to overnight for best flavor
Serve on Romaine, Bibb lettuce, or mixed greens. This is also insanely delicious as a wrap or sandwich. I also love with some grain-free crackers!