Soak in water all the ingredients except the salt for at least 4-5 hours and more if you have the time. Drain.
Blend the rice-dal mixture, in several batches, adding enough water to make a smooth batter that's runny enough to spread into a crepe, but thick enough to coat the back of a spoon. At this stage, you can leave the batter to ferment overnight, which helps make the dosas softer and also adds a ton of probiotic bacteria to your gut (see next step). But if you prefer the dosas unfermented, you can make them rightaway, without fermenting.
To ferment the dosa batter, cover the bowl with a plate or plastic wrap and place in a warm spot overnight or for 8-12 hours. Room temperature in summers in most places would work fine, but in cooler weather leave it in the oven with the light on (don't turn the oven on!). By morning, the batter will be puffy and bubbly, indicating the good bacteria are at work.
Add the salt before making the dosa and after the fermentation process is complete. To make the dosa, heat a cast-iron or non-stick griddle. Using a ladle with a rounded bottom, pour some batter into the center of the griddle and, in a quick but smooth motion, spread outward in concentric circles. Don't be afraid if you make holes: just add a small drop of batter to patch it. In this case, practice definitely makes perfect and trust me, you'll get the hang of it soon enough.
Pour a few drops of oil around the dosa's edges. This really helps give it that crispiness I love. Once the underside is golden brown, loosen the dosa gently from the skillet and flip over. If your griddle was hot enough to begin with, this step will be very, very easy.
Cook the other side for a few seconds, giving more time if your dosa is thicker. Serve hot with some sambar or chutney or both.