Wash the fish and marinate with turmeric and salt. Keep aside.
Soak the yellow and black mustard in warm water for 15 minutes. Drain the mustard. Make a smooth paste of the mustard with 3-4 green chillies, a pinch of turmeric powder and a couple of grains of rice. (If you choose, you can as well add one or two garlic cloves while making the mustard paste.)
Dissolve the mustard paste, remaining turmeric powder and salt in a little water. Keep aside.
Heat 2 tbsp oil, when smoking hot, add the kalonji seeds and green chillies and allow them to splutter. Now add the mustard and turmeric paste and cook for 2-3 minutes.
Add the fish and keep on simmer for 5 odd minutes. Till the fish is just cooked. You could throw in some more green chillies if you relish your heat :D
Finish with a hearty drizzle of mustard oil. Serve with hot rice.