Wash and soak the raw rice and black lentils separately for 2 hours. Add the fenugreek seeds to the raw rice for them to soak as well.
Drain the water and blend the soaked rice and the lentils separately in a mixer to a smooth paste using 1 cup of water each approximately.
Transfer the batter in the same bowl, add salt and mix well. The batter should neither be too thick nor too thin. Check the consistency, add water if needed.
Cover and ferment in a warm place for 10 hours.
After the batter is fermented, heat a non-stick pan, sprinkle little water and wipe it off gently using a cloth.
Pour a ladleful of the batter on it and spread it in a circular motion to make a thin circle.
Smear little oil or butter along the edges and cook on a high flame till the dosa turns crisp.
Fold over to make semi- circle or a roll. Serve hot with chutney or sambar.