Saute the onion in some butter to brown. When it just starts browning after about 15 minutes, add in the lamb. Cook until browned, removing fat if needed. Add in the spices and cook another 5 minutes. Remove from heat.
Meanwhile, mix all the other ingredients in a bowl. Add in the lamb and spice mixture after it cools slightly. Add more yogurt as needed. Allow to sit in the fridge for an hour or so before serving.