Sit the chicken on the onion and cook for 1 hour, then pour in ½ bottle dry white wine and cook for 30 minutes or until the chicken is cooked through.
Take the chicken from the tin and rest under foil for 15 minutes.
Discard the tarragon stalks and spoon off the excess fat. Put the tin on the hob and let the gravy bubble up.
Season, add the reserved tarragon, cook for a couple of minutes then serve with the chicken.