Preheat the oven to 140 °C.
1. Put the leg of lamb in a roasting pan and drizzle the lemon juice and olive oil over. Season with salt and freshly ground pepper. Massage the chopped garlic into the meat with your hands and scatter the chopped rosemary over. Put the garlic bulb halves next to the leg.
2. Transfer the roasting pan to the oven – do not cover with foil. Roast for 4 hours, then increase the oven temperature to 160 °C. Roast for another 2 hours or until the meat is browned and done. If the leg seems a bit dry during cooking, add a little water or even a dash of white wine.