Preheat the oven to 200 ° C. Cut the leek into rings and wash. Slice the tomatoes, season with salt and pepper. Wash the redfish fillet and pat dry. Cut the feta into thin slices. Heat sunflower oil in a large pan and sauté the leek for 5 minutes. Season to taste with salt and pepper. Put the leek in a greased baking dish and spread half of the feta over it. Spread the redfish fillet on top and season well with pepper and salt. Finally, pour the tomatoes on top. Stir the creme fraiche until smooth, season with nutmeg and spread over the tomatoes. Cover the casserole with the remaining feta slices and bake for 35 minutes.