Clean the fish & keep it aside
Dry roast red chillies, coriander seeds, cumin seeds, fennel seeds, methi seeds & transfer it into a mixer jar.
Heat pan, add 2 tsp of oil, add chopped onion( medium-sized) saute till onion become soft, then add freshly shredded coconut fry till it becomes golden brown & aromatic. Transfer this into a mixer jar.
Add ginger-garlic paste & turmeric powder, tamarind lemon sized & add little water to the mixer jar, grind into fine masala paste.
Heat kadai, add 3 tsp oil,add mustard seeds,then add onion(medium sized) & few curry leaves. Saute till golden brown.
Add ground masala paste & add masala water & bring the gravy to the required consistency, add onion(small-sized), chopped tomato, salt. (A typical Udupi- Mangalorean fish curry will have thin gravy, but you can adjust it according to your preference.)
Boil the gravy to about 6-7 minutes.
Add fish into the boiling gravy. Stir gently without breaking the fish slices. Boil the gravy again until fish get cooked well. This should take not more than 10 minutes on medium flame.
Serve this fish curry with boiled rice or white rice.