Wash and soak pearl millet, idli rice, fenugreek, split black gram separately for 4 hours. Soak flattened rice for 10 to 15 min.
Drain split black gram and keep the water in the freezer to chill for 10 to 15 min.
Grind split black gram using chilled water until its fine paste. Scrape the bowl of the grinder often.
Transfer ground paste to a bowl. Drain rice and grind to coarse texture. Add chilled water little by little and grind. Now add drained bajra and grind to a fine paste.
Remove half into the bowl. Add flattened rice, fenugreek, grind to a fine paste and transfer to a bowl.
Mix thoroughly with hand and clean the sides of the bowl. Let it sit in a warm place for 6 to 8 hours until its fermented.
Add salt and water according to the consistency needed. Mix and let it sit for 10 min.
Meanwhile heat a cast iron dosa tava or griddle. Sprinkle water if they sizzle and evaporate, the tava is ready. A hot tava is essential for dosa to cook evenly and attain that perfect golden brown colour.
Now pour batter using a katori with flat bottom, spread in circles from centre towards the edge. Reduce flame. Pour ghee and let it cook.
Fold crispy dosa in half and transfer to a plate. Rub the tava with half cut onion. Sprinkle water and repeat.