Heat oil in a flat pan or kadai.
Add the grated onions and fry on medium heat for 7-8 mins till light brown in colour.
Now add the masala paste and give a mix, add 1/2 tsp salt to it and continue to fry on medium heat for 3 mins till water dries up and oil separates.
Add the dried fenugreek leaves (Kasuri Methi) and fry 1 min on medium heat. Add the curry leaves and fry on low to medium heat for 2 mins.
Now add the tamarind juice, give a mix and then add 250 ml water. Cook it on medium heat till it comes to a boil.
Lower the heat and place the fried fish pieces side by side and cover & cook on low heat for 5 mins.
Remove lid and flip the fish pieces. Add the thick coconut milk, give a mix.
Cover & cook for another 10 mins on low heat till oil separates.
Garnish with the chopped coriander leaves, mix and simmer for a minute.
Serve with rice.