Poach chicken breasts in simmering water for 15 to 20 minutes until the internal temperature reaches 74 degrees Celsius.
Remove chicken from liquid and allow to cool completely before shredding or dicing into 1cm cubes.
In a large mixing bowl, whisk together Greek yogurt, lemon juice, Dijon mustard, salt, and pepper until the emulsion is smooth.
Incorporate the cooled chicken, diced celery, red onion, and chopped dill into the dressing mixture.
Fold ingredients together with a spatula until the chicken is evenly coated with the dressing.
Refrigerate the assembly for at least 30 minutes to allow flavors to meld before serving.