Gather all ingredients on a clean countertop.
Dice the cooked chicken into bite-sized pieces.
Chop the celery and red bell pepper into small cubes.
Halve the grapes and set aside.
In a large mixing bowl, combine the chicken, celery, red bell pepper, and grapes.
In a separate bowl, mix Greek yogurt and Dijon mustard until well combined.
Season the yogurt mixture with salt and pepper to taste.
Pour the dressing over the chicken mixture and stir gently to combine.
Add chopped walnuts for added texture and flavor.
Garnish with fresh dill before serving.