To skillet add minced shallot and cook over moderate heat, stirring, for 1-2 minutes, or until softened. Add vinegar, wine and tarragon. Deglaze pan, scraping up brown bits, until reduced by half. Add chicken stock (have heated so that it comes to the boil quickly). Bring to a boil. Strain sauce into a small saucepan and bring back to a boil.