Add the chicken to a large bowl (if from the stock recipe, add a tablespoon or two of hot stock to keep it juicy).
Stir in the lemon juice, salt, black pepper, and garlic powder.
Finely chop the celery, pickle, and scallions and add to the bowl, along with the chives and dill.
Stir in the mustard and tahini until well combined. Serve on toasted bread or in lettuce cups.
