Start with the marinade Squeeze the juice from the orange and lemon. Add the rest of the ingredients and process with a stick blender.
Tie the meat with kitchen string to form a neat shape with the fat on the outside and pour the marinade over it in a glass bowl. Rub well, cover and marinate for at least 2 hours (preferably overnight) in the refrigerator.
Remove the meat from the refrigerator and let it reach room temperature; meanwhile, preheat the oven to 160°C. Place the meat on a wire rack in a roasting pan with the other onion, orange slices and bay leaves. Season with salt and pepper and pour the marinade and about 250ml boiling water into the pan. Cover and roast for 3 hours.
Remove the meat from the oven, baste with the pan juices and roast uncovered for about 20 minutes to brown the fat. Let the meat rest before carving into thin slices.
Make a mouth-watering gravy by smearing about 1kg meaty pork bones with 60ml tomato paste and grilling them with a variety of hearty vegetables such as leeks, onions, carrots and celery in an ovenproof dish until well caramelised. Add a dash of white wine, beer or cider to deglaze the pan.
Put everything in a large pot then add parsley, bay leaves, a few peppercorns and about 2L of stock. Bring to the boil then simmer over low heat for at least 1 hour or until fragrant. Pour through a sieve into a smaller pot and add a generous splash of cream and any pan juices from your roast; this will ensure that you get all the rich flavours of your marinade in the gravy. Reduce until thick and glossy, or thicken with a little flour.