Wash idli rice well and soak separately. Wash urad dal and soak separately. Add 1/4 tsp of fenugreek seeds (methi seeds) into the soaked urad dal. Soak for at least 4 hours.
Place the soaked urad dal into the mixer jar and grind with little water. While grinding, the urad dal has to become fine, creamy and fluffy. The consistency should be like a very thick milkshake.
Remove the batter into a large bowl.
Then add soaked rice and cooked rice to the grinding jar and grind in batches with little water as needed. This time the paste should not be very smooth. Grind till you can feel the rice as fine a rava. Pour the batter into the urad dal batter. Mix well till both the batters combine very well with each other.
Keep the batter covered in a warm place to ferment overnight.
If the batter looks bubbly and slightly risen indicates that it is well fermented.
Stir the batter well and add 1 tsp salt and 1/4 tsp of baking soda (mixed with 1 tbsp water). Again mix well and keep aside for 5 to 10 minutes. Till then you can keep your steamer ready.
Grease the idli molds with butter and pour the batter into the molds.
Place the molds in the hot steamer and steam for 7 to 10 minutes.
Remove from the steamer, let it cool for sometime. Remove Idlis from the mold and serve hot with Sambar & Chutney.