Mix besan flour with enough water and make a batter (like dosa batter). Keep it aside.
In a kadai, put the oil and when it is hot, add mustard and when it pops up, add jeera, urad dhal, asafotida powder and fry till it become light brown. Add chopped onion, chillies and curry leaves and fry till the onion turn transperant. Add chopped tomato along with red chilli powder, turmeric and salt. Fry till the tomatos mashed well. Add two cups of water and bring to boil. When it starts boiling, add the besan flour batter and stir well till it become slightly thicker. Remove it and garnish with coriander leaves.
This can be served along with Poori, chapatti, dosa and idli.