In 5-6 qt. Saucepan, melt butter and sugar over medium-high heat; add quince. Turn fruit often until richly browned, about 15 minutes;remove slices as browned. Put quince in a 9x13 pan. Cover tightly with foil. If making ahead, chill up to a day. In the 5-6 qt. Pan, add lamb and onion. Cover and cook over high heat until mixture is juicy, about 5 minutes. Uncover; stir often until liquid evaporates and browned bits form in pan. Add 1/4 cup broth and scrape browned bits free. Boil until liquid evaporates and browned bits form again. Add another 1/4 cup broth; repeat step. When liquid evaporates, stir in remaining broth, minced cilantro, honey, ginger, cinnamon, cayenne. Bring to boil over high heat. Cover;simmer until meat is very tender when pierced, about 11/2 hrs. (if making ahead, cool, cover and chill up to a day. To continue, stir over meium heat until simmering.) Meanwhile, place covered quince in 300F oven until hot to touch, about 20 minutes(25 minutes if chilled). With a slotted spoon, lift meat from pan. Skim and discard fat from broth. On high heat, stir in cornstarch; stir until broth boils. Return meat and any juices to pan. On a platter, mound rice or couscous. Spoon stew around it. Arrange quince around stew; garnish with cilantro sprigs.