Dice or shred the cooked chicken. Halve the grapes, finely chop the celery, and finely dice the red onion.
In a large bowl, combine the chicken, grapes, celery, and red onion.
In a medium bowl, whisk together the mayonnaise, Dijon, lemon juice, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and stir gently to coat evenly.
Fold in the chopped nuts, if using.
Cover and refrigerate for at least 1 hour.
Give a final gentle stir and serve chilled on croissants, bread, lettuce wraps, or with crackers.