Take idli batter (which you prepare to make regular idlis) in a bowl.
Mix some water along with little bit of salt if necessary and mix it well.
Heat oil in seasoning pan and put mustard, cumin and gram. Add turmeric powder to it and remove from fire when it splutters and pour it over idli batter and mix well.
Grease idli trays and pour in the batter into idli moulds. Steam to prepare wonderful idlis in regular method.
