Urgent! Help,what is beef tenderloin?

Live in Belgium and am looking for beef tenderloin.Saw that it was translated as chateaubriand so I precipated towards a reputed butcher who had a large cut of chateaubriand.A large fatless rose colored mass.56. euros and 2 kilos of this later upon returning to the kitchen the resident romanian exclaimed that "This Is not a filet of boeuf" as had been translated by his dictionary.The point is this:looking chez Erma Bombeck' s Joy of Cooking the cut in question resembles a silverloin. All I want to do is paint it with mustard,herbs and garlic,bang it in the oven for 40 minutes or so.Romanian swears that after 40 minutes it will be inedible.Romanian is known for panicing unnecessarily. Panic is contagious.Do I chuck it in the freezer and hold out for mysterious muscle or can I use it as Bon Appetit recommended?Any illumination on this subject could really help both me and said romanian before I even out his flat head with my frypan. Thanks clarifications, or relevant information here.