Prepare the chicken by removing all meat from a rotisserie chicken or shredding cooked chicken breasts into bite-sized pieces and place in a large mixing bowl.
Whisk together mayonnaise, Dijon mustard, fresh lemon juice, salt, and black pepper in a small bowl until smooth and well combined.
Add finely diced celery and minced red onion to the bowl with the shredded chicken.
Pour the prepared dressing over the chicken and vegetable mixture.
Using a rubber spatula, fold everything together gently until all ingredients are evenly coated with dressing.
Add chopped fresh dill and parsley to the mixture and fold gently to distribute herbs throughout.
Taste and adjust seasoning with additional salt, pepper, or lemon juice as needed.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow flavors to meld together.
Stir gently before serving to redistribute any dressing that may have settled.
Serve chilled on croissants, bread, lettuce wraps, or over mixed greens and enjoy
