Marinate fish fillets with lemon juice, half a teaspoon of turmeric powder and salt. Set aside for thirty minutes. Heat two tablespoons of mustard oil in a pan and shallow fry fish pieces on both sides until slightly browned. Drain on absorbent paper and set aside.
Heat remaining oil in the same pan, add mustard seeds, onion seeds, whole red chillies and bay leaf, cook for a few minutes till seeds crackle. Add ginger paste, garlic paste and cook again for a moment. Add chopped onions and cook until slightly brown in colour.
Add mustard paste, red chilli powder, coriander powder and remaining turmeric powder. Stir and cook masala until oil starts separating. Add two cups of water and salt, bring to a boil and then add shallow fried fish.
Add slit green chillies and cook on a low heat until fish is cooked and oil starts floating on top. Remove and garnish with chopped coriander leaves and serve hot with steamed rice.