Poach the chicken breasts in simmering broth until
cooked through, about 16 minutes. Drain, cool, and
cut (or shred) into bite-sized pieces. In a small
bowl, combine the sour cream, mayonnaise, and
vinegar. Combine the chicken and dressing. (Can be
made ahead and refrigerated to this point one day
in advance.) Before serving, add the celery,
walnuts, tarragon, salt, and pepper to taste.
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