Rinse the rice and dal thoroughly. Add the fenugreek seeds and soak everything in water overnight or for 6-8 hours. Drain the water from the urad dal and rice, then transfer them to a blender jar. Add the cooked rice and grind everything into a smooth paste. Transfer the batter to a large bowl and mix well with a spoon until you have a thick batter. Cover and let the batter ferment in a warm place for 12 to 15 hours.

Once the batter has fermented, add salt and mix well. Grease the idli molds with oil and fill them with the prepared batter. Pour 1 cup of water into the idli steamer or pot and bring it to a boil. Place the filled idli molds in the steamer, cover, and steam on high heat for 10 minutes. Then reduce the heat. Open the lid and check if the idlis are cooked and fluffy by inserting a knife. Remove the idlis from the molds onto a plate. Repeat with the remaining batter.

Serve the hot idlis with sambar and chutney.