Chop onion, tomato, green chilli and coriander leaves finely.
In a bowl put Besan, red chilli powder, turmeric, asafotida, salt and mix well. Add required water and make batter like dosa batter. Add chopped green chilli and coriander leaves to the batter and mix it well.
Heat a non-stick tawa and pour a big ladle of the batter starting from the corner to the middle of the tawa (like making rawa dosai). Lift the Tawa and tilt in a circular motion so that the batter spreads uniformly. Sprinkle chopped onion and tomato on the top. Pour little oil around the dosa and allow to cook for few minutes. Flip it over and cook other side also and remove.