Wash and soak boiled rice and raw rice for 5 - 6 hours (or overnight). Grind with grated coconut, tamarind juice, red chilli powder, turmeric powder and salt. (If you add tamarind juice while grinding, then no need to add extra water). Grind until the batter is fine consistency in a mixer in batches.
Transfer this batter to another container / bowl and mix with asafoetida / hing and finely shredded cabbage and onion. Add extra water and salt if required and batter should be just like dosa batter and of pouring consistency. No need to ferment this batter.
Heat frying pan / dosa tawa. Apply little oil. Pour 1 ladleful of batter and spread it in a circle.
Cover with a lid cook for 5 -8 minutes on medium flame.
Turn other side and cook till golden colour. Colour of cabbage polo should be in golden colour with golden brown onions. Repeat for remaining batter. Serve hot with chutney / tomato sauce / coconut oil.