Wash and soak the urad dal , rice (and fenugreek seeds) separately in plenty of water for at least 3 hours.
Drain and grind the urad dal fine with just enough water to make a fine fluffy batter.
Transfer to a large container .
Drain the rice and grind smooth by adding just enough water to make a fine paste. Transfer to the container with the urad dal batter.
Add enough salt and a little water to mix the rice and urad dal pastes well together. Cover and set aside in a warm place to ferment overnight (8 to 10 hours under normal temperature).
Uncover mix well. Mix in a few tablespoons of water to make the batter into pouring consistency. Taste and adjust salt if needed .
Heat a tawa or flat griddle . When hot lightly brush the top with oil . Wipe of excess oil .
Pour a ladle of dosa batter at the center of the hot tawa. Spread the batter thin using the back of the ladle in circular motions.
Cook for 30 seconds or so till the sides begin to turn brown. Brush or sprinkle a few drops of oil on and around the edges of the dosa. Use a flat spatula or turner to flip the dosa . Cook for 20 to 30 more seconds and remove to a serving dish ,
Repeat with the remaining batter,
Serve hot with assorted chutneys , sambar or your favorite side dish .
You could soak the rice and dal together and grind together if the blender/grinder is powerful enough.
For detailed instructions go to https://oventales.com/plain-dosa/ .