Rub the pieces of fish head with 1 tsp turmeric powder and 1 tsp salt and pan fry till nicely browned on all sides. Keep aside.
In a dry pan, add the mung dal and roast over medium to high flame for about 5 minutes or until it is nicely coloured and starts leaving an aroma. Keep stirring continuously to avoid burning. Once roasted, remove from heat, wash with water (at room temperature) and drain. Keep aside.
Heat the mustard oil in a pan and add the bay leaves, green chillies, red chillies, cumin seeds, cardamom and cinnamon.
Add the onion and stir for a minute.
Add the ginger garlic paste and cook till the onion softens (about 2½ minutes)
Add the dal and stir for a minute.
Add salt (1½ tsp), turmeric powder (1 tsp), cumin powder and coriander powder. Stir for 2 minutes.
Add water, stir and bring to a boil.
Lower heat, cover and cook for about 20-25 minutes or till the lentils are just about to be cooked through. Stir every now and then.
Add the pieces of fish head and stir for a bit. Increase the heat to high ad cook for 2 minutes. Then, reduce heat to low, cover and cook for another 2 minutes.
Stir and break the bigger pieces of fish head.
Add the garam masala powder, stir for a couple more minutes and remove from heat.
Serve hot with plain rice.