Wash spinach thoroughly, squeeze to get rid of the water, chop roughly. Cube potatoes and par-boil. Slice onions and chop chillis. Now heat some oil in a saucepan and put in turmeric, mustard seeds and chilli powder. Fry till the mustard seeds pop, add the potatoes, onions, chillis and salt. Fry well for about 10 minutes and add the spinach and sugar. Stir well and simmer covered till spinach and potatoes are cooked. Uncover and cook to evaporate the liquid.