Heat the oil in a large flameproof casserole dish. Brown the chicken thighs in batches until golden, then transfer to a plate. Add the onions and mushrooms to the dish and cook for 5 mins until soft, then add the chorizo and cook for 2 mins more until the orangey oils are released into the pan. Stir in the flour for 1 min, then add the Sherry and bubble for 2 mins until most has evaporated. Return the chicken to the pan, add the stock, cover with a lid and simmer for 40 mins or until tender.