Heat the oil in a large non-stick frying pan. Fry the thighs for 8-10 mins, skin side only, until golden brown, then transfer to a casserole dish. Fry the drumsticks for about 5 mins each side and add them to the thighs.
Pour the stock over the chicken legs in the casserole. There should be enough stock to just cover the chicken, if not add a little water. Bring stock to the boil and cover, leaving lid slightly ajar. Cook at just below simmering point for 30-35 mins until chicken is cooked.
While chicken is simmering, drain oil from the pan. Heat the butter in pan and add onion. Sweat onion for 5 mins until soft, but not coloured. Turn up the heat, add the mushrooms, then fry for 3 mins until they soften and start to smell wonderful. Pour over the white wine, raise the heat to maximum and boil rapidly for 6-8 mins until reduced by two-thirds. Turn off the heat and leave until chicken has cooked.
Once chicken legs are cooked, strain stock into pan with the onion, mushrooms and white wine, bring back to the boil and reduce again by two-thirds until it is thick and syrupy. Pour in double cream, bring it to the boil, season if you want, then pour it over chicken. Heat chicken through in the sauce for 2-3 mins then turn off the heat and leave for a few mins before serving. This is such an aromatic and beautiful looking dish you should serve it straight from the casserole with the lid on.