Heat oil in a heavy-bottomed frying pan with a lid. Add the mustard and cumin seeds and fry until they pop. Add the potatoes, turmeric, chilli, ginger, salt and sugar.
Stir, cover and cook over a medium heat for 12-15 mins. You may have to add a little water if the potatoes start to stick. Add lemon juice and coriander leaves. Stir, lower the heat, cover and cook for 2-3 mins.
Good served with Cumin Chicken in a Thick Tomato Gravy.