1.Heat 2 tablespoons oil in a large saucepan on high. Brown lamb in batches, 2-3 minutes each. Transfer to a plate.
2.Heat remaining oil in same pan on high. Sauté onion, 3-4 minutes, until golden. Add cinnamon, coriander, garlic, turmeric and curry leaves. Cook, stirring, 1 minute.
3.Stir in lamb, stock and tomatoes. Bring to boil on high. Reduce heat to low and simmer, covered, 1 1/2 hours, stirring occasionally.
4.Add potato. Simmer uncovered for a further 25-30 minutes, until lamb is tender. Season to taste. Serve with couscous, topped with coriander sprigs, if you like.