1. Lightly brown the chicken in the oil and butter. Remove. Cook the onion in the pan until softened. Add the bouquet garni and chicken, cover and cook gently for 20 minutes.
2. Stir in the vinegar. Simmer, uncovered, until reduced by half. Stir in the tomatoes and tomato pur�e. Cover and cook for 5-10 minutes. Remove the chicken and keep warm. Discard the bouquet garni.
3. Boil the cooking juices to a syrupy glaze, stir in the stock and boil until well reduced and flavoured. Return the chicken to the pan and turn to coat with the sauce.