1. Wash 1/2 cup urad dal / black gram skinned until the water runs clear. soak it in enough water for 4 hours.
2. Wash 1/2 cup idli rice / sona masuri rice and 1/2 cup bajra / pearl millet together in a large pot. soak them in enough water for 4 hours. you can even soak bajra for longer if you prefer.
3. Drain water from both the pots. add urad dal to a blender jar along with some water. blend it until smooth and bubbly. (donot add much water as the batter will become too thin.)
4. Add bajra and rice to the same jar and pour some water. blend until the batter is smooth. batter must not be thin or runny as it will not ferment well.
5. Transfer this to a large bowl for fermentation. cover and set aside in a warm place for 4 to 8 hours depending on the climate.
6. Add 1/2 tsp salt and water as needed. bring it to a pouring consistency.
7. Heat tawa or a griddle. pour a ladleful of batter in the centre and spread it a dosa. sprinkle some oil and allow it to cook until the edges begin to leave the pan.
8. Flip and cook on the other side for 1 minute. flip back and cook until crisp.
Do not let the batter over ferment else it will turn sour.