To make the batter: Soak the urad dal and rice in separate bowls for at least 4 hours. Soak the fenugreek with the urad dal. Grind dal and rice separately with a little bit of water to create a smooth paste (the consistency should be that of pancake batter). Mix the two batters together thoroughly; add salt. Cover and set aside for a minimum of 12 hours to let the batter ferment, but no more than 18 hours.
The batter can be stored in the refrigerator for a few days; for best results, bring the batter to room temperature before using.
To make the dosa: Stir the fermented batter; ensure the consistency of the batter is thin enough to spread on a skillet like a crêpe. (A pancake-like consistency is too thick; add water to thin, if needed.)
Heat a skillet over medium-high. Spread ⅓ cup batter on a heated skillet in a circular motion, like a crêpe. Add a spoon of oil around the dosa as it cooks. When the sides can be lifted, flip the dosa to cook the other side. (The first side should be golden brown. Adjust flame, if needed.) Flip the dosa again to show the browned side, fold in half, and place on serving platter.