Poach chicken: In a large pot, arrange the chicken breasts in a single layer.
If using, place lemon slices and dill sprigs on chicken.
Pour water over the chicken breasts, covering by at least an inch.
Bring water to a boil, then reduce to simmer and cook for 10 or until the center of the chicken reaches 165° with an instant read thermometer.
If you don’t have a thermometer, check that the thickest part of the chicken is opaque.
Slice chicken into bite-sized pieces.
In a large bowl, combine chicken, apple, onion and celery.
In a medium bowl, combine mayonnaise and lemon juice and season with salt and pepper.
Whisk until combined.
Pour dressing over chicken mixture and toss.
Garnish with dill and serve.