In a wok, dry roast the semolina and rice flour on medium-low heat till they turn very light brown and a nice aroma of roasted flour comes. Keep aside and let it cool a little. To this, mix the yogurt, baking soda, ginger, cilantro and salt. Mix well. Add the buttermilk, a little at a time, mixing after each addition till the batter is slightly lighter than a pancake batter. Heat the oil for tempering, add the mustard seeds and once they splutter, add the curry leaves, green chillies and asafetida. Pour this on top of the idli batter and mix it in. Let it rest for at least 15 mins.