1. Put the pickling onions into a bowl and cover with boiling water. Leave for 2-3 minutes. Drain, then remove the skins from the onions using a sharp knife.
2. Heat the oil in a large, flameproof casserole. Add the onions and cook for a few minutes until golden brown. Add the chicken breasts and brown on both sides. Reduce the heat and cook gently, uncovered, for 15 minutes.
3. Put the remaining ingredients, except the thyme and cornflour, into a jug and mix together. Pour over the chicken. Add the thyme, cover and cook for 15-20 minutes until the chicken is cooked.
4. Remove the chicken and onions and place in a warm serving dish. Blend together the cornflour and 2 teaspoons water and stir into the cooking juices. Heat gently, stirring until it has thickened. Season to taste. Pour the sauce over the chicken and serve with boiled potatoes and a green vegetable.