1.) Combine wine, soy sauce, and brown sugar and stir until sugar is dissolved. Place tenderloin in marinade and refrigerate for at least 4 hours or overnight if you can.
2.) In a small bowl, combine mayonnaise, sour cream, chives and mustard powder. Mix well and chill until ready to serve.
3.) Preheat oven to 325 degrees F. Place meat and marinade in a shallow baking dish, and roast for 1 hour, basting occasionally. Let rest for a few minutes, then cut into 1/2 inch thick slices. Serve with mustard sauce.
Great with mashed potatoes and broccoli as the sauce tastes great with all of these!