Wash boal mach thoroughly under running water and then pat them dry.
Make a paste of the onion and garlic.
In a mixing bowl, make a marinade of onion paste, garlic paste, salt, turmeric powder, red chilli powder and 1/4 cup of mustard oil.
Add the fish to this and rub the marinade over the fish.
Take a non stick frying pan or a kadai. Heat it and then simply place the fish steaks along with the marinade in the kadai or frying pan.
Keep the heat at high for a minute and then reduce it to low. Cover it and cook for 8 - 10 minutes or until you see that the masalas are sticking and one side of the fish has cooked.
Flip the fish pieces gently and sprinkle some warm water if required. Stir around to check the masalas. If you notice that the raw smell of the onion paste and garlic paste has gone, then just cover again and cook the other side for a couple of minutes to fish to completely cook.
Once the fish has cooked from both sides and the masalas have all come together, add split green chillies. Sprinkle some more water if you need extra gravy. This is not exactly a fish with a lot of gravy. It's mostly made in oil and onion paste.
Add the balance mustard oil and srpinkle coriander leaves and stir.
Turn off the heat and serve boal mach bhapa with rice.