Wash and Clean the fish in a Colander under running water well. Sprinkle some turmeric powder and mix well. Keep Aside.
Take a small lemon sized tamarind and soak in warm water for 20 minutes.
Heat a Clay pot. Once hot, add gingelly oil.
Add mustard seeds, urad dal and fenugreek seeds(vendhayam).
Now, 2 green chillies, lots of curry leaves and chopped shallots. Cook till shallots turn soft.
Add turmeric powder, Red Chilli powder and coriander powder. Saute for few seconds.
Now add chopped tomatoes. You need to cook till tomato turns mushy and forms a gravy(thokku like consistency). Add required salt as it will help in cooking tomatoes faster.
Add Chopped raw mangoes and a cup of water. Cover the pan with a lid and cook for 3-4 mins.
Take the soaked tamarind and squeeze well. Discard the fibre. Now add the tamarind juice and 1 cup of water to the curry. Mix well.
Cook covered for 10 mins in medium flame. You will oil floating on the top. Taste the curry and adjust salt and spice.
Now add cleaned Nethili fish / Anchovies to the curry and let it cook for 5 - 6 mins.
Switch off the flame.