1.Roll and tie lamb with kitchen string at 5cm intervals. Using mortar and pestle, crush garlic, coriander root and stem mixture, spices and half the oil until almost smooth. Rub garlic mixture all over lamb; cover, refrigerate 2 hours.
2.In large frying pan, heat remaining oil; cook lamb until browned all over. Remove from pan. Add onion to same pan; cook, stirring, until onion softens.
3.Place stock, cinnamon and onion mixture in 4.5-litre (18-cup) slow cooker; top with lamb, drizzle with honey. Season with salt and pepper. Cook, covered, on low, 8 hours. Stand lamb 10 minutes; stir quince paste into sauce.
4.Thickly slice lamb, serve with sauce; sprinkle with chopped coriander.
5.Lamb can be covered and refrigerated overnight at the end of step 1.
6.Serve with couscous.