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Heat oven to 180C/160C fan/gas 4 and season the chicken all over. In a large flameproof casserole dish with a lid, heat the butter and oil until sizzling, then spend a good 15 mins slowly browning the chicken well all over. Remove from the dish and pop the lemon, bay and thyme in the cavity. Set aside.
Pour most of the oil out of the dish, place back on the heat and sizzle the chorizo for 5 mins until it starts to release its red oil. Throw in the potatoes, sizzle them until they start to colour, then add the garlic. Splash in the sherry, let it bubble down a little, then pour in the stock.
Nestle the chicken, breast-side up, among the potatoes, place the lid on the dish and cook in the oven for 1 hr 15 mins or until the legs easily come away from the body. Leave the chicken to rest for 10 mins, then scatter with parsley and serve straight from the dish.