Coat the chicken in the flour and seasoning.
Heat the oil in a large frying pan, add the onions and fry for 5 minutes until pale golden. Mix in any remaining flour, then scoop the onions out of the pan and set aside.
Heat half the butter in the frying pan, add the chicken and fry on both sides until golden. Arrange a thin layer of potatoes in the base of an ovenproof casserole dish, top with half the onions, then the chicken pieces. Add the apple and black pudding, then spoon over the remaining onions. Arrange the remaining potatoes in an overlapping layer on the top. Pour the stock over the top, then season the potatoes.
Cover the dish tightly and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 1½ hours. Remove the lid, dot the potatoes with the remaining butter and cook for 30 minutes more until golden brown. Serve in shallow bowls.