Cut mutton and pork in thin slices. Wash flank well and place on cutting board. Put mutton and pork on flank. Lay onion slices on top and sprinkle with seasonings. Roll up tightly and sew together. Put into a kettle of boiling water and simmer 3 hours. Remove and place in a low earthen dish; put a plate on top of meat to weigh it down. Put in refrigerator; let stand a day or two before using.