Soak the rice and urad dal separately for 2 hours and grind it to a paste (just like idly batter) it should be thick. To it add the chana dal flour and set aside for 10 mins. Then keep 1 or 2 tsp of oil in a pan and add the mustard and allow it to splutter the add the chana dal, jeera when the dal is light golden add the chopped green chillies, curry leaves, coriander leaves, red chilli powder, hing and remove from the heat. Add salt, the curd, and the grated radish and mix with the batter which you set aside, it should be just like idly batter. After mixing all this, you can add some more chana dal flour to make it thick.
Now heat up your appe tawa add a tsp of oil when hot pour the batter on each about a tbsp and turn when the sides are golden. Serve hot with any hot sauce, or any chutney.