In a small saute pan, bring the champagne vinegar and saffron to a boil for 1 minute. Pour into a bowl and whisk in the olive oil slowly as to incorporate it. Add the chopped preserved lemon and black olives. Season with pepper. I do not add salt to this because of the saltiness of the olives. This recipe makes a quart and will last for at least a month.
Preheat the oven to 500 degrees F.
Make sure the lamb leg is cleaned well and any hard fat is trimmed off. Make 8 slits in the leg and stuff them with the rosemary sprigs and garlic cloves. Rub the leg with the olive oil, dry and fresh oregano, lemon, salt, and pepper. Roast at 500 degrees F. for about 30 minutes. Lower heat to 375 degrees F. Add water to the pan if needed. Cook for about 3 1/2 hours.
Remove the lamb from the pan and let rest. Skim the fat from the pan juices and pour the juice onto the bottom of a platter. Slice the lamb into thin slices and serve topped with the preserved lemon sauce.