Combine the fenugreek seeds and ¼ cup water; cover and set aside.
Set a fine-mesh strainer in the sink and rinse the rice and urad dal with cold water until it runs clear. Transfer to a bowl and cover with 8 cups of cold fresh water. Cover and let soak at room temperature for 8 hours or overnight.
Preheat the oven to 200°F or set a dehydrator to 90°F. Drain the rice mixture, reserving the soaking liquid. Drain the fenugreek seeds and add them and the salt to the rice mixture.
Add 1⁄3 of the mixture and 1 cup of the reserved soaking liquid to a blender and purée until smooth. Transfer to a large bowl or storage container that can hold at least 14 cups (this will prevent overflow as the batter ferments). Continue blending the rice mixture in 2 more batches, adding it to the bowl. Stir (batter should be the thickness of pancake batter), then cover tightly.
To ferment in the oven, turn the oven off. Transfer the batter container to the warm oven or the dehydrator and let rest again for at least 8 hours, or overnight.
Once the batter has doubled in volume and looks frothy, it is ready. (If it has thickened too much, add a tablespoon of cold water at a time until pourable.)