Soak black and white mustard seeds in 4 tbsp water for 15 minutes. Add garlic cloves, 2 green chilies and make a fine paste in Preethi Zodiac. Marinate the fish with turmeric and salt for 10-15 minutes. Keep it aside.
Heat mustard oil in a pan.Fry fish slices both side at medium flame. Many like to fry super crispy. Though I don't like to overcook.
Once the frying is done, add 1 tsp oil if needed. Else in the remaining oil add Kalonji. Let it splutter. Add sliced green chilies.
Add chopped tomato, a pinch of turmeric to it. Cook for 2 minutes. Adjust salt. Add 1 cup water to the mustard paste. Add this to the pan and cook for 2-3 minutes at medium flame.
Once it starts boiling, reduce the flame. Add fried fish slices and cook for 5 minutes at slow flame. Garnish with chopped coriander leaves and serve hot. Best to pair with steamed rice.