Cut the leek and onion into thin rings, chop the garlic through a press or finely. Heat the oil in the pan and sauté the leek, onion and garlic over medium heat. After a few minutes, sprinkle the flour over the vegetables and roast them briefly, pour on the cream, stir well so that it binds without clumping, just pour in enough water until the sauce appears creamy. At the end stir in the parmesan and the spices to taste. Sauce should still be creamy. Put the fish fillet in a greased casserole dish and pour the sauce over it. In the oven at 200 degrees for about 20 minutes.