Marinate the fish with a pinch of salt, turmeric powder and red chili powder for 10-15 minutes.
Heat oil in a pan / kadai and temper with mustard seeds. After it stops spluttering add the cumin seeds and fenugreek seeds. Saute for a few seconds and then add the garlic. Saute for a few seconds more.
Add the onion, green chilies and curry leaves. Stir fry till light brown. Add the ginger-garlic paste and stir for few seconds more.
Now add the tomatoes and cook till it is completely mashed. Add all the dry spices and toss well. Add in the tamarind water and bring it to a boil.
Gently drop in the marinated fish and simmer, covered for 5-6 minutes. When done, switch off the flame and garnish with coriander leaves. Serve with plain steamed rice.
