Portions fish fillet halve, add salt and pepper, Place in the boiling milk and cook on very low heat until almost soft. Make a light roux from the butter and flour, dilute with milk, which is cooked tenderloin, and boil until a smooth sauce, which add salt, sprinkle with pepper, add the mustard and 150 g grated feta cheese . Cooked fillet portions are placed in an oiled baking pan, pour the sauce, sprinkle with grated feta cheese and baked in a hot oven or grill (about 5 minutes).