When you buy beef filet at the butcher, you normally have to clean it. The filet, or tenderloin how it is called, consists of 2 pieces: A long strip of meat with a layer of skin on one side, the actual filet, which ends in a broad piece of meat: This is the undercut, which contains some strands of skin and connecting tissue.
Start at the end of the long strip and sever the skin from the meat. Keep on pulling off the skin, helping with a sharp knife, until the long strip joins the broad piece. Here I normally cut off the filet which leaves me with the best piece of the cow. Now I clean the undercut from all skin. That results in some smaller pieces of meat which I normally use in a goulash.
The easiest way to cook the filet is to heat up some spoons of olive oil in a pan and quickly fry the meat from all sides. Once it starts to brown, add some water, turn down the heat and let it simmer for at least 2 hours. Add water whenever needed. Salt the meat just before the cooking time is finished. Before cutting let the meat rest for 15 minutes so the juices stay in the flesh. Slice the meat as thinly as possible and serve with mustard or any other condiment of your choice.
If you want to serve the beef in its own sauce, add diced onions, crushed garlic and carrots and any herbs you like at the beginning of the cooking. During the 2 hours cooking time the vegetables will almost dissolve and result in a nice, thick sauce.
If you have a pressure cooker, you can save a lot of time and gas. Just fry the meat from all sides, than add about half a liter of water and vegetables for the sauce. Close the lid and let it cook for one hour after the first whistle.
When you cook beef, always add the salt only during the end of the cooking time. If you salt the beef before cooking, it will remain hard and chewy.