Soak the green mung beans in water for more than 12 hours. Soak the urad dhal and rice for 2 to 3 hours. Peel the ridge gourd, wash well and chop them into small pieces.
Grind the mung beans alone in a blender until it's a bit coarse paste, then add urad dhal and rice with little water and grind it again. Finally add the peels, onions, ginger and green chilli grind it once again until the peel is ground completely. Pour it in a large bowl, add salt and stir well.
This batter doesn't need to ferment so you can use it immediately. Heat a non-stick pan, grease it lightly with oil, pour a ladle full of batter, swirl it with round tablespoon in circular form, like how you make normal dosai, pour a little bit of oil along the edges of the dosai.
When it is lightly brown on bottom, turn the other way round and cook for further 2 minutes.
Serve the dosai along with sambar, tomato chutney, coconut chutney or with mixed nut chutney and podi.