Marinate the fish with salt and 1/2 tsp turmeric powder.
Heat the mustard oil in a frying pan and fry the fish till light brown. Keep aside.
Reheat the same oil, add the chopped onions and green chillies, fry till the onions are soft.
Add the chopped tomatoes and cook over a medium flame till the tomatoes are soft and mushy.
Now add the remaining turmeric powder, red chili powder and salt. Cook over a moderate flame till oil starts to release from the masala. Splash a little water if needed, you wouldn't want the masala to burn.
At this stage, stir in the posto paste, mix well and continue cooking for 4-5 minutes on a moderate flame.
Add 1 cup of warm water, give it a nice stir and bring to a gentle simmer.
Gently add the fried fish, adjust the salt and cook for a further 5-6 minutes till the posto gravy has thickened.
Finish it with a generous drizzle of mustard oil and chopped coriander leaves. Serve hot with steamed rice.