Mustard-marinated pork tenderloin is a super-easy, make-ahead recipe that combines tender pork with mustard, garlic, and oregano. It's simple to make and perfect for entertaining. You have to like mustard to make this recipe because it uses two kinds of the spicy condiment, but cooking mustard will reduce the heat. If you love mustard, combine some dijon mustard with some sour cream, and use that as a sauce when you serve the pork. Make sure that you use pork tenderloin in this recipe, not a pork loin roast. These are two very different cuts. Pork tenderloin is like beef tenderloin. The muscle is in a place on the pig that doesn't get much use, so the cut is very tender. A pork loin is a much larger cut, and it needs long, slow cooking in a moist environment to get tender. Also, make sure that you buy an unflavored pork tenderloin. There are many varieties of flavored tenderloin on the market. Read the label every time you buy something packaged. If you are feeding more than four people, we recommend doubling this recipe. The pork is luscious and so flavorful people will probably eat more than the 1/4 pound typical for a main dish serving. Serve it with a green salad, perhaps tossed with some sliced mushrooms and avocados, and some roasted asparagus for a special meal. For dessert, brownies would be the perfect finishing touch.