In a wide bowl, combine gram flour, 1 cup water, whisk until you get smooth and lump-free slurry. Keep it aside.
Heat oil in a skillet on medium-low heat.
Once oil is hot, add mustard, jeera and let themcumin splutter for few seconds.
Now add urad dal, stir for few seconds until dal turns light brown.
Add curry leaves, green chillies, ginger and saute for few seconds.
Add onion slices, a pinch of salt, saute and cook for 2 minutes or until onion turns soft and translucent. Salt helps to bring out the moisture from onion and make cooking process faster.
Add turmeric and mix well.
Now add tomatoes and cook for 4-5 minutes until they start to soften.
Now add gram flour slurry, salt, 2 cups water, mix well and cook for 10 minutes or until it reach desired consistency. Keep stirring regularly to prevent the curry sticking at the bottom of the pan.
Turn off the heat, add coriander and mix well.
Serve delicious bombay chutney with poori, dosa, idli or chapathi.