Cut off as much fat as possible from the leg. Put onion, garlic, bay leaves, peppercorns, sugar and salt into saucepan, pour vinegar and water over them and bring to the boil. Let simmer 1 minute and set aside to cool. Put lamb into saucepan or pot into which it fits snugly, and pour lukewarm marinade over it. When cool refrigerate and turn the leg several times during the next 24 hours. The next day, remove leg from marinade and insert slivered garlic into the meat. Rub with salt and pepper, place in roasting pan and roast at 400 degrees until lamb is just browned (about 10 to 15 minutes). Add marinade ingredients, baste with marinade and reduce heat to 350 degrees. Roast for 1 1/2 to 2 hours, continuing to baste, until leg is done. A half an hour before roasting is finished, add the pitted, marinated plums or mushrooms with a few tablespoons of their liquid and baste. Serve with the sauce from the pan and white rice.
Wash and thoroughly dry plums. Prick all over with a sharp needles and place in a deep enamel or glass dish. Blend vinegar, sugar and spices in another enamel saucepan and bring to the boil. Then pour this mixture evenly over the plums. Let stand another day and the next day repeat this procedure. On the following day, pour plums and marinade into glass jars so that plums are completely covered by liquid.