Add all the ingredients mentioned under ‘dry mixture’ to the chutney jar of a mixer grinder.
Heat 5 cups of water in a pot and add tomatoes. Boil for 10-15 minutes till the tomatoes soften.
Once the tomatoes cool down slightly, mash them with your hands. You can also use a potato masher to do this. Keep them chunky and don’t mash them to a pulp.
In a pot or kadhai, heat oil and add mustard seeds. Once they start spluttering, add all the dried red chillies and curry leaves. Saute for about 30 seconds.
Add sliced onions and fry till light golden. Add smashed garlic cloves and saute.
Add the ground coriander masala, turmeric, salt and saute for 2-3 minutes.
Add the mashed tomatoes, all the reserved tomato water, rasam powder and tamarind water.
Bring this to a boil and simmer for 2 minutes. Turn off the heat, top with fresh coriander leaves and let this rest for 10-15 minutes.