Refrigerate idlis for 30 mins atleast.Slice it thin.Heat oil in kadai - add the slices in batches.
Shallow fry till golden brown.Repeat to finish, Remove and set aside.Now in the same kadai add mustard seeds let it crackle.Add curry leaves,red chillies - saute for a minute.
Add onion,ginger garlic paste,required salt saute until golden.Add chettinad masala powder.
Add fried idlies,give a quick toss.Cook covered for a minute.Switch off.Serve immediately.
Notes
Use leftover idlies for best results.Fresh idlies may crumble while frying so refrigerate for a hour and use it.
Saute onion until golden but it should not turn very soft.