Heat the oven to Gas Mark 5, 190°C, 375°F. Lightly oil a roasting pan, score the skin of the pork at frequent intervals (sometimes this has been already been done), and place the pork in the oven. Cook for 30 minutes per 500g (1lb) plus 30 minutes extra.
Add the onions and water for the final hour of cooking.
Cut the apples in half across and remove the core from each half, add to the roasting pan, baste with the cooking juices and cook for the final half-hour of roasting.
Remove meat from the oven and leave to rest for 10 minutes before carving.
Serve with roasted apples and onions and a selection of seasonal vegetables.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.