After you have soaked and drained the Blackgram dal, Fenugreek and rice, using a blender, with less than 1/3 cup of water, blend rice first and keep it separate.
Next, with less than 1/3 cup of water, blend blackgram dal and fenugreek together. Combine the two mixtures. The batter should not be too thick or too thin, similar to pancake batter.
Keep the batter in a big pot for at least 12 hours until it ferments and tastes a bit sour. If you do not get a sour flavor, add lime juice.
Heat a non-stick pan until it becomes medium hot and wipe the pan with a little oil. Take a small scoop of batter and pour it on to the pan. Spread it around and make a small circle of batter. Flip it over to the other side when one side is lightly brown, that's when air bubbles begin to appear. Reduce the heat to medium before you cook the next one.
If you find the dosai a bit dry, add 1/8 teaspoon of oil around the edges of the next dosai.