Grind onion, red chillies, cumin with little water to a fine paste. Soak tamarind in 1 cup water.
Add 3 tbsp oil in a deep sauce pan.
Add mustard seeds. Let it splutter.
Add one onion cut into roundels.
Add curry leaves
Saute till lightly browned
Add the fish slices
Add turmeric and toss gently closing with a lid
Now add the masala paste. Tamarind juice, salt to season
Add required water. Close the lid
Let it come to a boil.
Now simmer for 10 minutes
Open and add chopped coriander
Serve with steaming hot rice