Sprinkle shanks with sugar, salt and pepper on all sides; refrigerate for 6 hours. Dredge lamb in seasoned flour; shake off excess. Turn a large brazier or tile skillet to high heat; add oil. Place floured shanks in skillet; sear until dark golden brown. Remove shanks; turn down heat and add onions, celery and garlic. Cook until translucent.
Add wine and stock; return shanks to pan. Add olives, raisins, barberries, sugar, lemon, bay leaves, parsley and thyme; bring to a simmer. Cover; let simmer for 3 hours until fork tender.
When finished, remove shanks; cool separately. Strain sauce, cool and reserve. Use braising liquid to reheat shank and for plating.
To serve, place couscous in tagine; top with heated braised lamb shank and sauce. Garnish with barberries, lemon, garlic, olives, cilantro and oil.