Soak urad dal for at least 2 hours and add fenugreek seeds to it.
In another bowl, soak sooji for the same amount of time. Add just enough water for the sooji to be submerged.
Grind them both together along with salt and little water.
Cover the vessel and leave the batter to ferment overnight in a warm place.
The batter should rise and lighten like a pancake batter.
Now, take idli molds, spray cooking oil over it and pour a spoonful of batter into each mold.
Let it steam in the idli cooker for 8-10 minutes. Once its done, wait for the steam to escape.
Open the cooker and scoop out the idlis out of the molds.
In a pan, heat sesame oil. Temper mustard seeds and urad dal and fry till dal turns golden.
Now add curry leaves and half of the chopped cilantro and fry for 10-15 seconds.
Next, add chilli powder, turmeric powder, salt and fry for a few minutes.
Add the idlis and chat masala, mix it all well.
Ensure that the masala coats the idlis well.
Garnish with cilantro and serve.