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 taste.
Roll the potato and chickpea paste into a rectangle. Spread the filling down the middle and fold over in half, making a large pasty. Pinch the three edges together to seal in the filling. Wrap in a sheet of oiled greaseproof paper and place in the refrigerator for 2-3 hours. When ready to serve, cut it into slices.