Soak the black and yellow mustard seeds in warm water for 15 odd minutes. Drain from water, make a smooth paste with 4-5 green chillies and a pinch of turmeric powder.
Add 3/4 cup of water to the mustard paste, mix well. Strain the mustard paste and keep the mustard water aside.
Smear the fish with a little salt and turmeric powder. Heat 2 tbsp oil and shallow fry the fish. Keep aside.
Add 1 tbsp of oil to the same pan. Throw in the kalonji and remaining green chillies. Allow the spices to splutter.
Gently add the tomato purée, sprinkle the turmeric powder, red chilli powder and a little salt, give it a good stir and patiently cook over a medium flame till oil starts to separate from the masala.
Now gently place the fried fish in the curry, pour the mustard water and cook over a low flame, 7-8 minutes or so.
Finish with a generous drizzle of mustard oil and a good handful of chopped coriander leaves.
Serve hot .