Heat oil in a earthernware kadai. Add in mustard, dry red chillies and fenugreek seeds and let them sizzle.
Add in shallots, garlic, ginger, curry leaves and turmeric powder. Saute this for 3 mins or so.
Now lower the flame and add in spice powders. Saute this on low heat for around 5 mins. The spices will get toasted and colour will darken.
Now add in tamarind pulp and water. Add in salt and bring it to a boil.
Once it reaches a boil, add in fish and mix well. Add in some curry leaves
Cook this partially covered for 5 to 7 mins on medium high heat. The fish will be cooked by this time.
Now take it off the heat and cover with a lid. Let it sit for 2 to 4 hours for the flavours to develop. You can serve immediately as well, but it will taste wonderful if it sits for a while.
Now serve this with boiled tapioca or rice.