Wash the fish pieces and pat them dry. Make a marinade of the spices mentioned and rub on the fish pieces and leave aside for 10 to 15 minutes.
In a kadai or a frying pan, heat mustard oil. When the oil is smoking hot, gently place the fish pieces in the pan. Reduce the heat to medium and cook the fish for not more than a minute on each side. We just want a mild colour to the fish and bhetki cooks too fast. Don't discard any leftover marinade. We will use that in the gravy. Take the fish out of the pan and keep aside. It is such a soft fish that you have to handle it very carefully.
In the same oil, add cinnamon and cinnamon myrtle. Stir fry for a few seconds and add chopped onion. On medium to high flame, fry the onions till they are golden.
Add the leftover ginger and garlic paste and continue cooking till the raw smell has gone.
In a small bowl, take 1/4th cup of coconut milk and mix coriander powder, red chili powder, turmeric powder, quandong, and crushed mountain pepperberry and make a slurry.
Add this slurry to the onions and on high heat continue cooking the spices while stirring continously.
Add any leftover marinade to this and sprinkle some water if required.
When the raw smell of the spices has gone, place the fish pieces in the pan.
Take the rest of the coconut milk and mix lemon myrtle in it. Add this to the fish.
Gently with the help of a spatula, mix all the spices. If needed, add a little bit more water.
Add salt and sugar as required. You will need sugar to balance the tartness from the quandong powder.
Add slit green chilies and let it simmer for 3 - 4 minutes. Then turn off the heat.
Serve immediately with steamed rice.