Take a bowl and add in the ragi flour and rice flour. Add in the chopped onion, green chillies and coriander leaves. Add in the hing (asafoetida), salt and cumin seeds. Take half a cup of plain yogurt and whisk with one cup of water until its smooth and lump free. Add it to the bowl. Mix well. Rest it for 15 minutes. The batter will now be very thick. Add in one more cup of water. Mix well. The batter should be very thin. Heat a dosa griddle till hot. Take a ladle of the ragi dosa batter and pour all over the pan. Do not spread with a spatula. Just pour evenly over the griddle surface. Sprinkle a little oil and cover the dosa griddle with a lid. Cook for 30-40 seconds. Flip once and cook for 30-40 seconds more. Ragi Dosa is ready. Serve hot with your favorite chutney.