Gather all your ingredients on a clean countertop.
Dice the cooked chicken into bite-sized pieces and place in a large mixing bowl.
Chop the celery and finely dice the red onion, then add to the bowl.
Halve the grapes and add them to the mixture.
In a small bowl, combine mayonnaise and Dijon mustard, mixing well.
Pour the mayonnaise mixture over the chicken and vegetables.
Season with salt and pepper to taste.
Gently fold the ingredients together until well combined.
Garnish with fresh parsley and serve chilled.