1. Put the fish in a nonmetallic bowl. Sprinkle with the salt and turmeric. Set aside.

2. Purèe the onion, ginger, and garlic in a blender or a food processor. Heat a large wok or deep frying pan over high heat until hot. Add the oil and swirl it to coat the bottom of the pan. Reduce the heat to medium. Add the mustard seeds, cardamom pods, and chile and stir until the mustard seeds sputter and "jump." Add the onion paste and cook, stirring often, for 3 minutes until it just begins to color.

3. Stir in the coconut milk. Boil for 2 minutes. Reduce the heat to medium-low and add the fish, with any juices in the bowl. Spoon the sauce over the fish and simmer for 2 minutes, occasionally basting the fish with the sauce, taking care not to break up fish.

4. Add the shrimp to the pan and simmer for a 2 minutes more, or until the shrimp turn opaque and the fish flakes easily. Add the lime juice and season to taste with salt and pepper. Serve hot, garnished with the cilantro leaves and lemon wedges.