1.In a medium-sized bowl, combine lamb and spices.
2.In a large deep saucepan, heat butter and half the oil, Cook the lamb, in batches, for 2-4 minutes, or until browned all over. Remove from pan.
3.In the same pan, heat the remaining oil and cook onion, ginger, garlic, paprika and cayenne over low heat, stirring for about 2-3 minutes, or until onion softens.
4.Return lamb to pan with stock, undrained tomatoes, bay leaves and cinnamon. Add yoghurt, 1 tablespoon at a time, stirring well between each addition; bring to the boil. Reduce heat; simmer, covered, about 1 and a half hours, or until lamb is tender.
5.Meanwhile, to make raita: In a small bowl, combine all ingredients. Season to taste.
6.Remove lamb from stove. Sprinkle lamb with nuts and chilli. Serve with raita and warmed naan bread, if desired.