Boil potatoes, drain, peel, cube.

Heat the oil and stir in mustard seed. When they crackle, add onions, green chilies, coriander leaves, and potato cubes.

Cook on moderate heat and keep turning to coat the mixture with the spices.

When potatoes are done, remove from heat and stir in salt.

Put yogurt in a bowl, add potato mixture, refrigerate for an hour.