Preheat the oven to 200° C. In a large frying pan gently heat the oil. Finely chop the onion and fry. Chop each mushroom in half and then into thick slices. Fry with the onion until both are softening, being careful not to allow the onion to crisp. Add the chestnuts to the pan to heat through. Transfer to an ovenproof dish. Turn up the heat and brown the chicken on both sides. Place in the dish with the vegetables, and add the remaining ingredients, seasoning well. Cover and bake in the oven for about 30 minutes, until the chicken is cooked through. Serve the meat with the pan juices and vegetables spooned over and some green vegetables alongside.