Soak moong dal in water for 5 - 6 hours.

Drain the water from the soaked moong dal. In a blender add moong dal, ginger, green chilies, cumin seeds, salt and grind to a smooth batter consistency by adding water little by little. 

Optional: In a separate pan, heat oil, add chopped onion, green chilies and cook until the onion becomes soft. Keep it ready. In a hot skillet, pour a ladle of batter and spread into a thin circle. Drizzle oil on sides and cook until the dosa turns crispy. Top the dosa with the onion mixture and serve.