Heat oil in a pan and fry the marinated fish on medium to low heat for 2 to 3 minutes carefully flip over and fry on the next side and remove.
Into the same pan add the onion, green chillies, ginger, garlic, and curry leaves and fry for 2-3 minutes, until softened, on medium heat.
Now, add coriander powder, and turmeric powder. Cook for a minute, until fragrant.
Then add the thin coconut milk and stir to combine. Bring the mixture to a simmer.
Add the fried fish and also the sliced tomatoes. Stir gently to combine and cook for 5 minutes, until the tomato has softened and the mixture has thickened.
Finally, add the thick coconut milk and vinegar. Cook for 5-6 minutes, until the fish is cooked and the stew is heated through.
Serve hot with steamed rice and a side dish of your choice.