Unroll roast, and trim fat. Reroll roast; secure at 2-inch intervals with heavy string. Make 10 (1/2-inch-deep) slits in surface of roast; stuff garlic halves into slits.
Combine juice and the next 6 ingredients (juice through bay leaf) in a large zip-top plastic bag. Add the lamb; seal and marinate in refrigerator 8 to 24 hours.
Preheat oven to 450°.
Remove lamb from bag, reserving marinade, and discard bay leaf. Place the roast on a broiler pan, and insert a meat thermometer into thickest portion of roast. Sprinkle the roast with 1/2 teaspoon salt. Bake the roast at 450° for 20 minutes, basting once with 1/4 cup reserved marinade. Reduce oven temperature to 400° (do not remove the lamb from oven); add quartered oranges, and bake 1 hour or until thermometer registers 145° (medium-rare) to 160° (medium). Let the roast stand for 10 minutes before slicing. Combine the pan drippings, the remaining marinade, and cornstarch in a small saucepan; bring to a boil, and cook 1 minute. Stir in parsley. Serve sauce with lamb.