If you haven’t already, cook the chicken and let it cool. Dice it into bite-sized pieces.
In a large bowl, combine the diced chicken, celery, grapes, and nuts (if using).
In a small bowl, whisk together the mayonnaise, Dijon mustard, and lemon juice until well combined.
Pour the dressing over the chicken mixture and gently toss until everything is evenly coated.
Add salt and pepper to taste. Adjust seasoning as needed.
Cover the bowl and refrigerate the chicken salad for at least an hour to let the flavors meld together.
Before serving, garnish with chopped parsley if desired. Serve on a bed of lettuce, in a sandwich, or with crackers.