Soak green gram and rice together for atleast 8 to 10 hours in enough water.
Grind with onion, ginger and green chillies.
Add salt at the end of grinding.
For filling: Heat oil in a pan and fry cumin seeds, green chillies with diced onions for 2 to 3 min and keep it aside.
Heat a heavy dosa pan and make dosa by spreading from center.
Add a tsp of oil around and roast. Sprinkle the onion mixture on top and press with a flat laddle.
Roast in midium flame till the dosa becomes crisp.
Fold and serve hot with ginger chutney or onion chutney.
Note: Do not turn over the dosa after adding the filling.