Cook the potatoes in salted boiling water for 15-20 minutes, or until very tender. Meanwhile, heat the oil in a ovenproof casserole and add the onion and tomato puree. Cook on a medium heat for 5 minutes.
Add all of the spices and fry for 1 minute. Add the lamb, season well and cook, stirring occasionally for around 8 minutes, or until the lamb has cooked through. Add the pine nuts and parsley, then take it off the heat.
Set the oven grill to medium. Drain the potatoes, add ¾ of the butter and all the buttermilk, season well and mash the potato. Spoon over the lamb, dab the last of the butter on top and grill for 5-8 minutes, until brown.