Wash, then soak the lentils and rice separately in water overnight.
The next day, grind the soaked lentils and rice with water and the rest of the ingredients to form a coarse batter. The batter needs to be about the consistency of pancake batter.
I like to use an Indian style mixer-grinder for grinding, but a food processor or powerful blender may work. You may need to grind in 2-3 batches to prevent overloading your mixer.
Mix batter well, it does not require fermentation and therefore is now ready to use.
Preheat a griddle on medium heat and dot the surface lightly with oil (only for the first). Pour a ladle full of the batter in the center, then spread out into a circle using the rounded part of the ladle.
Dot the surface of the dosa with oil (especially in any holes on the surface). Cover and let it cook for 3-4 minute Check the underside - it should be a rich golden color.
When the underside is done, flip the dosa and let it cook uncovered for 2-3 minute
Fold in half and remove to a plate. If you can, serve it right away instead of stacking - it tastes best when still crisp from the griddle. If you're the cook, I guess you're out of luck though.
Repeat for the rest of the batter.
Once the lentils and rice are ground, you may add chopped raw onions, chopped spinach, fenugreek leaves, or diced serrano into the batter. I'm fussy and don't like any of them.
Serve hot with unsalted homemade butter, Indian pickle, chutney powder or sambar.