Mash the potatoes and chickpeas and pound to a paste (or purée in an electric mixer). Bind with the oil to make a smooth, pliable paste. Mould it into a ball and put aside.
Now make the filling. Soften the onion with the garlic in the oil. Stir in the pine nuts, currants and sugar and cook for 2-3 minutes. Add the spices and bind with the tahin. Season to t