Cut the meat into neat chops and remove the bones and surplus fat. Place the meat in a casserole so that it is about half full. Thinly slice the onion and place on top and barely cover with gravy or stock. Season carefully. Cover and cook gently in a moderate oven (350° F., Gas 4), for about 1½ hr. Roll the suet pastry to the shape of the casserole—but slightly smaller. Lay the pastry on top of the meat, replace the lid and cook gently for 1 hr. longer. Before serving, divide the pastry crust into suitable portions.