If not already cooked, poach or grill the chicken breasts until fully cooked (internal temperature of 165°F). Allow to cool, then shred or dice into bite-sized pieces.

Finely chop the celery and red onion. Halve the red grapes. This adds texture and flavor to the salad.

In a large mixing bowl, combine the mayonnaise, Dijon mustard, lemon juice, salt, and black pepper. Stir until smooth and well combined.

Add the shredded chicken, chopped celery, red onion, and halved grapes to the bowl with the dressing. Gently fold the ingredients together until everything is evenly coated.

Transfer the chicken salad to a serving bowl. Garnish with chopped parsley. Serve chilled or at room temperature, either on its own or in a sandwich.