Score the pork with narrow lines. Mix the onion with the sage, salt, mustard and pepper and rub the mixture well into the meat. Wrap the joint in greased greaseproof paper and roast in a covered tin in a hot oven (425° F., Gas 7) for 10 min. and then reduce heat to moderate (350° F., Gas 4) for the remainder of the time. Allow 25 min. per lb. and 25 min. over. About ½ hr. before serving, remove the paper and lid and continue cooking to crisp the crackling. Serve the apple sauce and gravy separately.