1 Place leg of lamb on a rack in a roasting pan. 2  Preheat oven to 325 degrees and bake leg of lamb uncovered for 30 minutes per pound or until internal temperature reaches 120 to 130 degrees at the thickest point. Preheat oven to 325 degrees and bake leg of lamb uncovered for 30 minutes per pound or until internal temperature reaches 120 to 130 degrees at the thickest point.