Boil the meat in salt water for about 30 minutes. Cook the onions in the butter and some water for about 5 minutes, add the rice, meat, pepper and mint and cook for 5 more. Add 2 cups of the lamb stock and boil 20 minutes. Add the rice mixture and the meat in a cooking pan, put in the oven and bake 30 minutes at 375F. Beat the eggs with the yoghurt, pour over the lamb and rice and bake for another 10 min.