Pre-heat the oven to 200°C. If the joint is not already scored, take a sharp knife and make shallow cuts across its skin at about 3 cm intervals.
Mix the coating ingredients together and put into a medium-large freezer bag, together with the joint. Shake it about and rub in all over the meat through the bag.
Slice the onion into 4 thick rounds, place in a roasting tray and rest the coated joint on them with the skin side upwards.
Pop into the oven and cook for 20 minutes, then turn down to 180°C.
The total cooking time should be 35 minutes per 500g plus another 35 minutes. For our 2.2 kg joint this worked out at about 110 minutes i.e. 20 mins at 200°C and 90 mins at 180°C.
When the times up put the joint on a carving dish to rest covered with foil for about 15 minutes. In the meantime put the tray on the stove and make some gravy from the roasting juices – add a Tbsp of cornflour, apple juice or cider, chicken stock or vegetable stock and reduce, stirring constantly, until you get a thick sauce. Strain it into a jug.