Wash potatoes and boil in plenty of water till tender.
Drain, peel and cut into 5 cm pieces.
Heat oil in a pan and sprinkle in mustard seeds and dal. When mustard seeds start sputtering, add onion, green chillies, ginger and curry leaves, and fry till onion turns translucent.
Sprinkle in turmeric and salt, and mix in potatoes and 3/4 cup water.
Cover pan and cook for 10 minutes over moderate heat.