Heat the oil in a large saute pan. Brown chicken on all sides in the oil, and remove from pan.
To the oil remaining in the pan, add the onion. Fry lightly for 2 minutes, then add the garlic. Deglaze pan by stirring in the wine, which should lift off the residue of the flour from the bottom of the pan. Add the tarragon, and salt and pepper to taste. Return the chicken to the pan. Cook over low heat for about 45 minutes, or until the chicken is well done.
When ready to serve, thicken the sauce with a cornstarch and water mixture; often this is not necessary as the initial flour coating thickens it naturally. Stir in the sour cream.
