In a pan heat oil. add mustard seeds, cumin seeds. once they stop splattering. Now add the mashed potato, salt if needed and mxi well.
Now add chili powder, turmeric powder, coriander powder, little water as needed. Cook this until the raw smell is gone and masala is very thick. Remove from heat and keep it aside. NOTE: sprinkle water and cook to avoid burning.
Heat water in idli pan. On the idli plate cover the idli mould with a cloth. Now pour 1/2 ladle of idli batter in the mould . Now place a spoon of masala over the 1/2 filled mould. Now pour 1/2 ladle idli batter over the masala. do this for all the mould.
Now steam the idli until they are cooked and serve.
Goes well with Chutney.
When you cook the masala make sure it not running has to thick like we do for masala dosai.
To test for idli cooked usually we do finger test. i.e. dip your finger in water and poke the idli. if finger comes clean then idli is ready. In this case you cannot use finger test you have to rely on timing.