Preheat the oven to 120°C/Fan 100°C/ Gas ½.
Season the chicken flesh (not the skin). Chop the onion. Finely slice the peeled garlic. Coarsely chop the tomatoes. In a small saucepan, bring the white wine to the boil and let it boil for 10 seconds before removing from the heat. This will remove some of the alcohol but keep the freshness of the wine. Reserve.
In a large heavy-based casserole dish over a medium heat, heat the oil and then sear and lightly colour the chicken pieces for 8–10 minutes. Transfer them to a plate. Add the onion and garlic to the casserole and sweeten them over the heat for 4–5 minutes but do not brown them. Spoon out some of the fat, pour in the vinegar – it will give off a slightly aggressive smell (you might cough), but when that aroma has faded, that’s just right, and then reduce it down to a syrup.
Add the salt and peppercorns and the boiled white wine. Whisk in the mustard, add the chopped tomatoes, whole tarragon sprig and sage leaves. Pour in the water and return the chicken to the pan. Bring to the boil, cover with a lid and transfer to the oven to cook for 35–40 minutes, stirring occasionally.
Test the chicken is cooked by using a temperature probe or taking out a leg and cutting down to the bone to check it’s not pink in the centre. Taste and adjust the seasoning. Garnish with the chopped parsley or chives and serve at the table from the pot. Eat with crusty baguettes or with creamy mash and wilted seasonal greens.