To make the mix for the onion rava dosa, mix all 3 flours in a large bowl.
Add in the sour buttermilk and add the water carefully to make a thin batter. This will be much thinner than the regular dosai batter.
Heat 1 tsp oil in a small pan or a tempering ladle. Fry cumin and mustard seeds. Once they pop, add the curry leaves and green chillies. Stir for a few seconds and add it to the prepared batter.
Season the batter with salt. Keep aside for 30 minutes. Just before preparing the dosas, mix in the finely chopped onions and coriander.
Oil a non-stick tava / pan. Once hot enough, pour one ladle of batter at a time from a slight height, so that it bubbles and gets a netted appearance all around. Take a second ladle of batter and pour around this to increase the size of the dosa. If you are a beginner, then make smaller sized dosas with just one ladle of batter.
Drizzle some oil along the edges of the dosai (traditionally sesame / gingelly oil is preferred). On a medium flame, the first side should take 1 1/2 - 2 minutes. Carefully flip the dosai over to the other side, keep for a minute of so until it has turned golden brown and crisp and it is ready to eat. Serve hot with spicy tomato chutney.
Notes
You can replace part of the rice flour or all purpose flour with ragi flour to make Ragi Rava Dosa.