Wash and soak green moong and urad dal in enough water, separately for 4-5 hours. Soak methi seeds along with urad dal.
After the required time, drain the excess water from both the dals. Take urad dal in a mixer and grind it to smooth batter adding ¼ cup of water. Transfer it to another vessel.
Now take green moong and grind it to a smooth batter adding around ¼ - ½ cup of water. Transfer the batter into urad dal batter container. Mix both the batter well
Let this sit on the kitchen counter for 8-10 hours for fermenting. I left it overnight. If the climate is very cold in your place, try to keep the batter vessel inside your oven with the lights on to help in fermenting.
Next day morning, add salt and mix well. Grease the idly moulds with gingely oil/any vegetable oil. Pour a laddle of batter in each mould and steam cook for 10-12 minutes.
Once done, take the idly mould out, let it cool for a minute or two. Using a spoon unmould the idlies on to a serving plate.
Serve hot with Chutney or Sambhar. I served them today for breakfast with Masoor dal sambhar