Optional step - Steam or microwave the potatoes until fork tender.
Heat oil in a heavy bottomed pan; add mustard seeds and once they start popping, add the curry leaves and cook for 30 seconds.
Next add the onions, green chilies and cook till onions are lightly browned around the edges.
Add the potato cubes, cover and cook till the potatoes are tender, about 12~15 minutes, stirring occasionally. This step would go faster, if the potatoes are pre-cooked.
Stir in turmeric, ground coriander, ground cumin and salt. Sprinkle little water, if the mixture looks too dry. Taste and adjust seasoning. Serve with rice or roti.