Rinse idli rice 3 to 4 times until water runs mostly clear. Soak rice with poha in enough water for 4 to 6 hours.

Rinse urad dal and fenugreek seeds separately, then soak together for 4 to 6 hours.

Drain urad dal-fenugreek mix. Grind in a wet grinder or high-power blender using cold water little by little until very light, fluffy, and smooth.

Drain soaked rice-poha mix and grind to a slightly grainy paste. Add water gradually; batter should be smooth but not runny.

Combine both batters in a large deep bowl. Mix by hand for 1 to 2 minutes to aerate; this helps stronger fermentation.

Cover the bowl loosely and ferment in a warm place (about 28 to 32 C) for 8 to 12 hours, or until batter rises and looks airy.

Check fermentation: batter should increase in volume, smell pleasantly sour, and look bubbly. If not fermented enough, rest longer in warmth.

After fermentation, add salt and gently fold once or twice. If batter is too thick, add 1 to 2 tablespoons water; keep it thick-pourable.

Grease idli molds lightly with oil. Do not over-grease, or idlis can become oily on the surface.

Bring water to a boil in the steamer first. Fill each mold about 80 to 90 percent with batter and place plates in steamer.

Steam on medium heat for 10 to 12 minutes. Avoid opening lid in the first 8 minutes or idlis may sink.

Check doneness with a wet toothpick or knife; it should come out clean.

Turn off heat and rest idlis for 2 minutes. Then remove with a wet spoon or butter knife for smooth unmolding.

Serve hot idli with chutney and sambar. For soft reheating later, steam leftovers for 2 to 3 minutes before serving.