Rub the tenderloin with Dijon mustard and sprinkle with your favourite steak spice. In a cast iron pan or heavy-bottom pan, sear the tenderloin for 2 minutes on all sides in butter then finish in a pre-heated 300°F oven on a parchment paper-lined baking sheet for 20 to 30 minutes, depending how you would like it cooked. Remove from oven and let rest for 10 minutes before cutting in thin slices.
Mix all ingredients together and refrigerate.