Marinate the fish fillets with lemon juice, half a teaspoon of turmeric powder and salt and keep it aside for at least 30 minutes.
Heat 2 tablespoons of mustard oil in a pan and shallow fry the fish fillets on both sides till slightly browned and keep them aside.
Heat the remaining oil in the same pan, add mustard seeds, onion seeds, red chillies and bay leaf, cook for a few minutes till the seeds start crackling.
Add ginger paste, garlic paste and cook for a minute.
Add chopped onion and fry till they turn golden brown in colour.
Add the mustard paste, red chilli powder, coriander powder and the remaining turmeric powder and stir until the oil starts separating from the masala.
Add two cups of water and salt, bring to a boil and add the fried fish.
Add the slit green chillies and cook on a low heat until the fish is properly cooled and started floating on the top.
Remove and garnish with freshly chopped coriander leaves and serve hot with steamed rice.