If you plan to use the water in which the rice & dal is soaked, then rinse rice & dal separately and then soak in enough water. This is very nutritious and also helps with fermentation during cold season.
First, soak Idli rice separately in enough water. Soak Whole skinned black gram / Ullundhu / Urad Dal with fenugreek seeds in enough water in another bowl. A minimum of 5 to 6 hours of soaking time is required. Overnight works too.
Rinse & Drain
Rinse and drain if you plan to use regular water if not reserve the water drained from the rice & dal.
Grind
First, grind black gram / urad / ullundu along with fenugreek seeds in a wet grinder using about 1/2 cup water. It might use about 1.5 cups of water approximately to grind. Add little by little every 5 minutes.
Grind for about 20 to 30 minutes or until its light and fluffy.
Test to see if its ground to the right consistency. Add a little of batter to a bowl filled with plain water. Batter should not sink nor melt but has to float. It should be not sticky & light. This is the right texture & consistency and once that is attained, transfer the batter.
Second, grind the idli rice adding 1 cup water. This should drink about 2 to 2.5 cups of water for total of 4 cups of rice. Add little by little and grind for about 15 minutes.
Test to see if this has reached the right stage. The ground rice batter should feel like fine rava like texture. When pressed between finger should mash up well. This is the right texture & consistency and once that is attained, add this to the ground Urad batter.
Mix
Add salt preferably rock salt / crystal salt / coarse kosher salt / Kallu uppu. This salt is free of iodine and other chemicals. Helps with fermentation.
Mix well using your hands. Warmth of your hands will help to ferment, so do wash your hands and then mix the batter.
How to ferment
Cover the batter with lid and place in a warm spot for 12 hours to ferment. (Applies only if you live in a warmer climate)
If you live in a colder place, then preheat oven to 350 F and switch off! You may keep the oven light on if you wish for added warmth.
Keep the batter inside the oven to ferment. In summer months it should ferment in 8 to 10 hours and in winter it might take up to 15 hours.
The batter will rise once it ferments. Always use a wide big pot to ferment the batter as it might need extra room when it rises. It will definitely rise for 2 to 3 inches from its original place almost doubling in quantity.
After fermentation, the batter will rise with tiny air pockets. This is an indication that its fermented well and is ready for making idli or dosa.
If the batter ferments too much, it might smell and taste sour. This batter won't last long and will continue to over ferment with time. Make sure to finish soon by making Uttappams.
To make Idli
Do not mix the batter too much, gently mix and pour a ladleful of batter scooped out of the pan. Undisturbed tiny air-bubbles in the batter helps in making soft spongy idlis. Steam cook for 10 minutes over high medium flame. Switch off and let in rest in the pan for few minutes before you scoop out. Taking them immediately will result in torn idlis with rugged top and sides. Making idlis the traditional way… using light cotton cloth ”thunni idlis” are always the best.
To make Dosa
Mix the batter with little water and bring the consistency slightly thin. Adding about 1/8 teaspoon of sugar to your dosa batter makes nice red golden color dosas. This is purely optional. Preferably use an iron pan to make dosas.
Scoop some batter the swirl over the hot tawa and drizzle some oil. Once it turns brown on one side, flip to the other side and cook for few seconds. Flip again, fold and serve.
How to store batter
If you plan to store the batter for more than 5 days then do not add salt nor ferment. Just refrigerate the ground batter which should stay good for 10 days. Bring it room temperature, add salt and then ferment when required.
Fermented Batter stays good in refrigerator for a week
Possible mistakes & fixes.
If the batter doesn't ferment well - idli won't come soft and will look flat after steaming. Dosa will be very chewy and white. Add little baking soda or eno to rise it a bit and try to finish the batter soon
If the add too much water while grinding urad dal - idli will have a shiny feel / slimy with rubbery texture. Dosas will be very chewy. Using less water will yield a thick sticky batter will make the idlis go hard after steaming and dosa will be hard and batter will stick more to the pan. This will still work for dosa but trying making thick kal dosa.
If you add to much water while grinding rice will make the batter runny and the water will separate and settle at the bottom after fermentation. The batter will still work for making dosa but idlis will look very thin and slimy. Grinding rice with less water, will yield a very tight batter that will result in hard idlis and cardboard like dosas.
All these not so good batter can be used for making rava paniyaram (just mix some with rava, soak and then make kuzhi paniyaram ) punugulu (using grated veggies) bonda (using rava and veggies) or wheat dosa (mixing wheat flour)
Wet Grinder or Mixie
Using a wet stone grinder will yield best results. Large quantity can be made plus the batter will be ground evenly without too much heat.
Mixie / Blender will still work but the texture and batter consistency will look a little different. If using a regular blender or mixer jar to grind... refrigerate or freeze the jar ahead of time to chill the empty jar and then use it for grinding to avoid overheating of the motor. Or use ice cold water to grind the batter. Too much heat while grinding will result in killing the bacteria and the batter won't ferment well.
Only a little can be ground each time and need to adjust water quantity to keep the blades running.
To yield soft idlis use little soaked aval / poha while grinding rice.