1. In a bowl, combine the fish, 1 teaspoon of the turmeric, and the salt.
2. In a coffee grinder or a spice grinder, combine the black and yellow mustard seeds and poppy seeds. Grind them until smooth. Transfer to a bowl.
3. In a food processor, combine 1 green chile, the coconut, and water. Work the mixture until it is a paste. Transfer to a bowl. Add 1/2 teaspoon turmeric and generous pinch of salt.
4. In the food processor, work the onion and ginger to a rough paste. Transfer to a bowl. Stir in the yogurt, 1/2 teaspoon turmeric, and generous pinch of salt.
5. In a large skillet over medium-high heat, heat the mustard or sunflower oil. Add the fish and cook for 3 to 5 minutes until the fish turns opaque. Add the halved green chile. Continue cooking for 30 seconds. Add the mustard paste, onion paste, and mustard oil, if using.
6. Cover and cook over medium-low heat for 7 minutes. Uncover and continue cooking for 2 minutes or until the fish is cooked through. Garnish with slivered chiles and serve with rice.