Heat a pan and dry roast rava (sooji) over low flame, taking care not to brown it. When you hold it in your fist, you should feel the heat. Transfer to a bowl and keep aside.
To the same pan, add semiya (vermicelli) and dry roast till golden brown over low flame. Transfer to the same bowl and set aside.
Heat a tadka pan or the same pan for tempering. Add oil and when it gets warm, add mustard seeds and let it splutter.
Add channa dal, finely chopped green chilies, ginger (if using), curry leaves and cook for 5 seconds till the dal gets golden brown. Set aside.
To the roasted ingredients, add the tempering, curd, salt and mix well. It should be very thick. Set aside for 10 - 15 mins.
Grease idli moulds and drop spoonfuls of batter into the indents till full. If the batter is too thick like a dough, add 2 to 3 tbsp water and whisk till it comes to dropping consistency.
Steam for 10 mins or till a skewer inserted comes out clean.
Remove the idlis with a spoon.
Serve these rava semiya idlis piping hot with chutney and sambar.