Peel and roughly chop the onions and cook them very gently in a frying pan (skillet) with duck fat or beef dripping and a little salt and pepper, for about 15 minutes, or until the onions are limp. Remove from the frying pan and place in a casserole dish.
Cut the liver into slices about ½" (1cm) thick and, in the same frying pan, fry quickly on both sides over a high heat until browned on both sides (one minute on each side should be enough). Add to the onions in the casserole dish. Fry the sausages in the same manner - just to brown them, but not to cook them. Cut each sausage into two or three pieces and add to the casserole dish with the liver and onions. Cut the bacon into strips about 1" (2.5cm) wide and add to the casserole (they will cook as the casserole cooks).
Drain off all but about 1 tablespoon of fat from the pan then stir in the flour, seasoned with salt and freshly-ground pepper. Cook gently till browned, stirring all the time. Add the beef stock (** you can make this from a stock cube if you wish) and stir until the liquid boils. Re-season with a little more salt and pepper if required, then transfer to the casserole dish. Cover the dish and transfer to a preheated moderately hot oven (Mk 6 - 400ºF - 200ºC) for about 30 - 40 minutes. Serve just with Creamed Potatoes.