Pat the fish steaks dry with kitchen paper. Rub them, on both sides, with ¼ teaspoon of the salt, ¼ teaspoon of the cayenne and the ¼ teaspoon turmeric. Set aside for 30 minutes.
Heat 4 tablespoons of the oil in a saucepan over a medium heat. When hot, put in the fennel and mustard seeds. As soon as the mustard seeds begin to pop (this just takes a few seconds)