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