This mustard based spicy fish curry is pretty much a default way of cooking fish curry in eastern region of India. The dish is pungent and spicy, so it may be an acquired taste. I absolutely love them.
Mix the mustard paste, remaining turmeric powder, salt and green chillies in 1 1/2 cups of hot water.
Add a few drops of mustard oil to the sauce. Bring the sauce to a boil in a saucepan and add the fried fish to it.
Simmer until fish is tender and sauce has thickened slightly. Add lemon juice. Take off the gas and garnish with coriander.