Add in the gram flour to a large bowl. To this add 2 cups water. Whisk well or mix with your hand, ensuring there are no lumps. Keep aside.
Heat oil in a pan over medium heat. Add mustard seeds, urad dal & chana dal. Once the mustard seeds splutter & the lentils turn golden, add in slit green chillies, minced ginger & curry leaves.
Sauté for a minute & then add in thinly sliced onions. Add a little salt to soften the onions. Add in the turmeric powder Sauté till the onion become soft & translucent But, do not brown them.
Now, add the chopped tomatoes to the pan. Cook till the tomatoes turn mushy.
Add the gram flour slurry to the pan. Mix immediately to avoid lumps. Add in 1 cup water & a little coriander at this stage, it gives a nice flavour. Adjust water to get the desired consistency.
Cook on medium heat for 3-4 minutes or till the chutney starts to boil & thickens. Stir intermittently.
Add in asafoetida & just cook for a minute or two and turn off the heat. The Bombay Chutney will thicken. Cook until you get the desired consistency. But, it should still be quite runny. Remember that it will thicken further on cooling.
Finally garnish with some coriander and squeeze in half a lemon.
Serve immediately with idli, dosas, puri, chapati or roti. Garnished with finely chopped fresh coriander & a dash of lemon if you wish.