Dice the cooked chicken into about ½" cubes and place in a large mixing bowl. Season the chicken lightly with salt and pepper if necessary and then add the grapes and walnuts. Mix to combine and set aside.
In a separate small bowl, combine the mayonnaise, sour cream, diced celery, red onion, parsley, dried tarragon, lemon juice and lemon zest. Stir until well combined. Pour the dressing over the chicken and mix until everything is coated. Cover the bowl and refrigerate for at least 1 hour.
After the salad has chilled for at least 1 hour, give it a stir and season to taste with salt and pepper. Enjoy!
Notes
Our recipe calls for dried tarragon but you can use a fresh variety if you double up on the amount!