Chicken Salad Sandwich

In a large pot, arrange the chicken in a single layer. Place lemon slices and dill sprigs on chicken and pour water over it, covering by at least an inch. Bring water to a boil, then reduce heat and simmer until cooked through, 10 minutes. Let rest 10 minutes, then slice into 1" pieces. In a large bowl, combine chicken, apple, onion, and celery. In a medium bowl, whisk together mayonnaise, Dijon, and vinegar and season with salt and pepper. Pour dressing over chicken mixture and toss. Garnish with chopped dill and serve on a baguette with lettuce.