Take a large bowl and soak 1 cup urad dal and 1 cup aval for 4 hours by adding enough water.
Take another bowl and soak 2 cup idli rice for 5 hours.
Drain off the water and transfer urad dal - aval mix into the blender, blend to smooth and fluffy batter adding water as required. Transfer the batter to a large mixing bowl.
Now, blend the soaked rice after draining off the water completely to slightly coarse paste as of rava texture. Transfer the rice batter to the same mixing bowl, mix well, making sure the batter is combined well.
Cover and rest in warm place for 8-10 hours or till the batter ferments and doubles.
The batter will double and well-fermented air pockets will be present.
Add 1 tsp salt to the batter and mix gently without disturbing the air pockets.
Now grease the idli plate with oil, scoop the batter into idli plate greased with oil.
Place in steamer when the water in the steamer boils and steam for 12 minutes on medium flame or till a toothpick inserted comes out clean.
Recipe by Kerala Cooking Recipes at https://www.keralacookingrecipes.com/idlisoft/