Wash the fish fillet, pat dry, season with salt and drizzle with lemon juice. Rub with 1/4 teaspoon cayenne pepper and the turmeric. Let it steep for 30 minutes.
Heat 4 tablespoons of oil in a pan and fry the fennel and mustard seeds in it. Put the onions and garlic cloves in the pan and brown. Add the cumin, 1/2 teaspoon salt and 1/4 teaspoon cayenne pepper. Add the diced tomatoes, the toasted cumin and the garam masala. Cover and simmer over low heat for 15 minutes.
Heat the remaining oil in a second pan and fry the fish briefly on both sides. When serving, pour the tomato sauce over the fish.