Mix ragi flour, rice flour, curd and salt with water to make a thin pourable batter. Set aside for 30 minutes. Add onions, cumin seeds, coriander leaves, ginger and green chillies to the batter.
To make the dosas, heat a tawa (or flat griddle). Ladle the batter on one corner of the tawa and spread into a thin circle by lifting and rotating the tawa, causing the batter to spread. This type of dosa is difficult to spread using a ladle as it has a tendency to tear easily. Gently grease with oil on the edges and the middle. Cook for a few minutes and flip over to the other side. Cook a few more minutes, transfer to a plate and serve immediately. Dosas are best eaten hot and crispy right off the tawa.