Pull the roasted chicken into bite-sized pieces by hand — irregular shapes are better than uniform cubes. Discard skin or save it for crisps.
In a large bowl, whisk together the mayonnaise, Dijon, and lemon juice until smooth.
Add the chicken and celery. Fold gently to coat.
Add the parsley and chives. Fold again until herbs are evenly distributed.
Season with flaky salt and black pepper. Taste and adjust — the lemon should balance the richness without overpowering.
Refrigerate for 15–30 minutes before serving to let flavors meld.
Spoon the chicken salad into a shallow bowl or onto toasted bread.
Finish with a crack of black pepper and a pinch of flaky salt.