Grind together onion, pepper, cumin seeds, garlic, coriander seeds and red chili coarsely.
Add tomato, curry leaves, turmeric, and salt and pulse for 5 seconds to make a paste.
Heat oil, add mustard seeds. When the mustard seeds splutter, add the ground paste.
Fry for a couple of minutes till the raw smell of the ground paste vanishes.
Add tamarind water and keep the flame low.
Allow the liquid to simmer. Don’t let the charu boil. When it starts foaming and is just about to boil, switch off the flame.