Rinse lamb and place in a 2-inch-deep 11- by 17-inch pan. Bake in a 500° oven until well browned, turning shanks over once, about 15 minutes. Discard fat from pan.
Meanwhile, in a 10- to 12-inch nonstick frying pan over medium-high heat, frequently stir onions with oil until dark gold, about 15 minutes.
Add coriander, turmeric, ginger, cumin, and saffron to onions and stir for 1 minute. Stir in broth and honey.
Pour broth mixture over lamb. Cover pan tightly with foil. Bake in a 350° oven until meat is very tender when pierced, 1 1/4 to 1 1/2 hours; after 30 and 60 minutes, spoon pan juices over lamb.
With a slotted spoon, gently transfer shanks to rimmed dinner plates; keep warm.
Skim and discard fat from the pan juices.
Place pan on 2 burners over high heat and boil juices, stirring often, until reduced to 2 cups, 12 to 15 minutes.
Spoon juices equally over the lamb shanks.