Soak tamarind in 2 cup of water and get the juice. Crush the tomato and add to the juice.
Grind coconut, 1/2 chopped onion into a fine thick paste by adding little water as possible.
Crush the garlic cloves, green chilies.
Heat oil and add mustard seeds, cumin seeds, black pepper, fenugreek seeds. Once the mustard seeds stop splattering add onion, curry leaves, green chili, garlic cloves until they are sautéed. i.e. onion turns translucent.
Now add Chili powder, coriander powder, turmeric powder, salt and sauté until raw smell is gone. NOTE: don't burn it.
Now add the ground coconut paste and sauté.
Now add tamarind juice, 1 cup of water and bring to boil. Continue cooking until the kuzhambu becomes thick (oil starts to separate).
Add the fish and cook until the fish is cooked.
Finally add coriander leaves. cook for a minute. Remove from heat and serve.
Best consistency is when oil separates and float over the kuzhambu