First of all for making stuffing heat oil in a non stick kadai then put mustard seeds and cumin seeds in hot oil and let them crackle.
Now add in split black gram, garlic chilli paste, curry leaves and saute them well for little time, now add in onions and cook till it become light golden brown.
Now add in salt, turmeric powder and saute for little time, now add in boiled potatoes and mix well with all the spices and turn the heat off, make small patties or balls from this stuffing and keep aside.
Now grease the idli stand or mould, now put salt as per taste and eno in idli batter and mix well, when you add eno to batter it will doubled.
Now put about 1 tbsp batter in idli mould then put one potato stuffing ball or patties then again put 1 tbsp batter on patties so it can be nicely covered.
Now put the idli stand in steamer and cook it for 10-12 minutes, after 10-12 minutes turn the heat off and take out idli stand from steamer and let it cool for 5 minutes.
After 5 minutes un-mould the stuffed idli from stand using a knife or spoon and serve hot with coconut chutney and enjoy.