This is an unashamedly luxurious main course, because this cut of beef (known as fillet of beef in other countries) has become so expensive. However, it is economical in the sense that not a scrap needs to be wasted of this lean and tender cut. Leftovers can always be used up in delicious dishes and make wonderful beef salads or even sandwiches. It's best to roast this only to "rare" - still edge-to-edge reddish-pink. Although I give a timing method, it is still best to test the meat about 2/3rd through roasting time, as ovens differ and the thickness of tenderloins differ. A rare roast will easily yield to finger or spoon pressure, and become increasingly unyielding as it cooks through. Some people prefer a very hot oven, some prefer it as in this recipe. I roasted this one to medium -- pink -- because my guests preferred it that way. Tenderloin this way is an easy way to put a smart main dish on the table because it can be prepared in advance, timed well, and stuck into the oven. Just remember that it is very important to let the finished roast "rest" for 15 minutes (in a lukewarm oven) to ensure the meat is juicy. One last point: a small piece of tenderloin/fillet is not worth roasting, and is better when sliced into cutlets and pan-fried. This recipe is for the largest one you can find! Quantities are guesses as it's not that important. For the mustard, I simply scooped out Dijon mustard with a spoon until the roast was covered. NOTE that the cooking time given is very approximate, as it all depends on your timing.