Season 1½ tablespoons of the flour on a plate. Coat the beef and kidneys with the flour. Heat 2 tablespoons of the oil in a flameproof casserole and fry the meat in batches until browned on all sides, lifting out with a slotted spoon on to a plate.
Heat the remaining oil in the casserole and fry the onions for 5 minutes. Tip in any flour left over from coating. Cook, stirring, for 1 minute. Blend in the stock.
Return all the meat to the casserole and bring to a simmer, stirring. Cover and cook in a preheated oven, 160°C (325°F), Gas Mark 3, for 1½ hours.
Mix together the remaining flour, baking powder, suet and a little salt and pepper in a bowl. Add enough of the measurement water, mixing with a round-bladed knife, to make a soft dough. Roll the dough out on a floured surface to a round the same diameter as the casserole.
Stir the oysters, parsley and Worcestershire sauce into the casserole and increase the oven temperature to 200°C (400°F), Gas Mark 6. Lay the pastry dough over the filling and brush with beaten egg to glaze. Bake for about 25 minutes until the pastry is pale golden.