Mix gram flour with water and whisk to make a lump free batter.
Heat vegetable oil in a pan.
Once the oil is hot, add mustard seeds, cumin seeds, hing and curry leaves and let them crackle for a few seconds.
Add onion and green chillies and cook until onion turns translucent.
Add tomatoes and cook for 2-3 minutes.
Now add turmeric powder, red chilli powder, garam masala powder and salt to taste and cook for another minute.
Add the besan mixture and cook on medium heat until the besan thickens.
Add fresh coriander and mix well.
Bombay chutney is ready to serve.
To make a lump free batter, add little water in besan first and mix it well until smooth. Then add the rest of the water.Use a wire whisk to make the batter.If for any reason you have to store it, add some water before heating it as it thicken once cooled.