Step 1 Rub the fish with one-half teaspoon of salt and 1 teaspoon turmeric. Set aside in a medium bowl.
Step 2 Soak the cumin and mustard seeds in 4 teaspoons of warm water for 10 minutes, then put the seeds in a blender with the ginger and onions. Blend, using a little water as needed (we used one-quarter cup) to get a smooth, thick paste.
Step 3In a large wok or karai, heat the canola oil over high heat until shimmering, then reduce the heat slightly to medium-high and fry the fish pieces, turning occasionally until crisp and brown, about 3 minutes on each side. Remove the fish to a plate and drain the oil, reserving 2 teaspoons of the oil.
Step 4Add back the reserved oil to the pan and heat the pan over medium-high heat. Add the onion seeds and fry for 15 to 20 seconds, then add the onion paste. Reduce the heat to medium and add the remaining 2 teaspoons of turmeric, red chile powder, ground coriander and potato wedges. Add a quarter-cup of water, cover loosely and cook until potato is almost done and a knife pierces the wedges fairly easily, about 20 to 25 minutes. Check the pan every few minutes, adding a quarter cup of water each time the contents begin to look dry, and stirring gently but often so nothing burns. Add the cauliflower, eggplant, green chiles, salt to taste and another quarter-cup water to thicken the sauce. Cover loosely and cook for about 10 minutes, stirring occasionally, until the cauliflower and eggplant are cooked.
Step 5Arrange the fish in the sauce and cook for another 3 to 4 minutes. Add more water if needed to thin the sauce, and turn the fish pieces over once, but do not stir. Sprinkle with cilantro. Serve immediately.