In a Dutch oven with a tight-fitting lid. Add chicken stock and water to cover the chicken. (Don't add the chicken.) Boil some extra water in a teakettle.
64 oz Chicken stock
Add scallions and peppercorns to water, cover, and bring to a rolling boil. Turn off the heat and add the chicken pieces to the hot water. If needed, add boiling water from the kettle to cover chicken with water by 2 inches. Replace the lid and let chicken rest in the hot water for about 2 hours. Do not turn the flame back on: The pot will retain enough heat to cook chicken thoroughly and safely. (To test, cut into 1 piece of chicken and check the meat near the bone. If it is still pink, return the pot to low heat, bring the water to a simmer and simmer 10 minutes more.)
3 lbs Boneless, skinless chicken thighs, 2 ea Scallions, cut into thirds, 1/4 tsp Black pepper corns
Lift chicken out of the pot. Pat the meat dry with paper towels, then cut or shred into small bite-sized pieces and transfer to a medium bowl. Discard the liquid.
In a small bowl, whisk together the juice of ½ lemon, mayonnaise, and sour cream. Whisk in mustard. Taste and adjust the seasonings and thickness.
1 ea Lemon, cut in half, 2/3 cup Mayonnaise, 4 Tbsp Sour cream, 1/2 tsp Dijon mustard
Add celery, onion, nuts, herbs, and salt and pepper. Toss gently but thoroughly. Refrigerate, covered, for at least 4 hours. Taste and add salt and pepper as needed. Sprinkle with herbs before serving.
3 ea Celery ribs, cut into 1/4" slices and diced, 1/2 cup Onion, diced, 1/2 cup Walnuts, broken into bite sized pieces, 3 Tbsp Parsley (or tarragon or chives), chopped