Wash the fish fillets, pat them dry with kitchen paper. Sprinkle the fillets with salt to season and keep aside.

Blend together the red onion, ginger, garlic and green chilli in a spice blender or coffee grinder, untill it becomes paste-like.

Heat a heavy-bottomed pot with high sides on medium heat on the stovetop. Add the vegetable oil, followed by the blended onion paste. Sauté for a minute, stirring continuously.

Add the red chili and turmeric powders and stir again, sautéing for a minute more.

Now add the canned tomatoes or tomato puree, and mix well with the paste, letting it cook for a couple of minutes. I like to use a whisk to break down the tomatoes, so that it cooks evenly.

Now add the coconut milk and using a stick blender, blend together the gravy so that it has a smooth consistency. This is entirely optional, you can avoid this step if you like a little texture in your gravy.

Add the souring agent of your choice – I used kokum, but you can add tamarind (whole or paste) as well. Add the salt to season the gravy and stir to combine.

Add the fish to the gravy, and let it simmer on low heat for 7-10 minutes, until the fish is cooked.

In a small saucepan (I like to use this tempering pan) on the stovetop, bring the coconut oil to heat on medium heat. Now add the mustard seeds and let them heat until they start to crackle. Add the curry leaves and pearl onions to the pan. Let them heat until the pearl unions turn translucent, around 3 minutes.

Top up the fish curry with the tempering just before serving.