Soak both the rice together with fenugreek seeds and dhal separately in water for about 6 to 8 hours. Wash and grind rice and dhal separately to a fine paste. Mix together with salt and make a thick batter. Leave it for at least 8 hours to ferment.
Heat tawa. Wipe it with oil. Mix the batter and add little water if required and make it to a thick dosa batter. Pour one big ladle of batter in the tawa and slightly spread it to a thick small round dosa. Pour few drops of oil around the dosa. Close with a lid and cook on medium flame for 2 to 3 minutes. Remove it. No need to flip it over.
This dosa is mostly prepared on Deepavali days in some households. It is served with rich non-veg/veg kurma or veg sagu and other gravy type side dish. Can also be taken with usual chutney/sambar.
You can sprinkle little Idli Milagai Podi (Idli chilli powder) on top of the dosa with gingelly oil and pack it for lunch or for long travel. It stays soft for longer time.