Wash and soak the lentils and rice overnight.
The next day, rinse and drain all the water. Transfer the lentils and rice into a blender and add a little water and blend into a fine paste. It may be a little grainy. That is not a problem.
Transfer this paste to a non metallic bowl and cover. Leave it to ferment for 24 hours at least (This may change if you live in a warm country) The next morning you will see bubbles in the batter and the batter would have doubled in volume and have a sourish smell.
Season the batter with salt and add a bit of water to thin it down. The ideal batter consistency is similar to a crepe / pancake batter. It should be flowing, yet not watery. Something like coating consistency (the batter flows off a spoon but leaves a coating of batter on the spoon)
Now to cook the dosa. Heat the non stick pan or cast iron pan over a medium heat.
In small bowl, add the oil and gather it together with the spatula, ladle and batter mix next to the stove. You want everything ready!
Once the pan is hot, season the pan by brushing the oil onto the surface. Not too much oil though, just enough so that the dosa doesn't stick to the pan.
Sprinkle some water on the pan, if it sizzles then you are ready to make the dosa.
Pour about two thirds of a ladle of batter (for a classic 10 inch pan about 100ml of batter) down onto the center of the pan and very quickly swirl the batter using the bottom of the ladle in a circular motion outwards into a circle to the edge of the frying pan. Do not press down, its a gentle motion to encourage the batter into a circle.
Increase the heat a little and drizzle about half a teaspoon of oil onto the dosa.
After a minute or so, you will see the surface of the dosa is almost cooked.
Now gently place the flat spatula under the sides of the dosa to pull it up. If it is not lifting, reduce the heat and allow it to cook for another minute or so.
Once the bottom of the dosa is crispy and golden brown and lifts off easily, fold the dosa in half. If using any filling add the filling before folding over the dosa.
Between cooking each dosa, clean down the pan with a kitchen towel. Repeat the process above.