1/3 cup red wine
1/3 cup soy sauce
2 tablespoons light brown sugar
2 pounds pork tenderloin
1/3 cup mayonnaise
1/3 cup sour cream
1 1/2 tablespoons mustard powder
1 tablespoon minced fresh chives (optional)
Combine wine, soy sauce, and brown sugar in a large resealable plastic bag. Place tenderloin in bag, and refrigerate overnight, or at least 8 hours.
In a small bowl, combine mayonnaise, sour cream, mustard powder; mix well. Mix in minced chives if you wish. Chill until ready to serve.
Preheat oven to 325 degrees F (165 degrees C). Place meat and marinade in a shallow baking dish, and roast for 1 hour, basting occasionally. Temperature of meat should register 160 degrees F (73 degrees C). Let rest for a few minutes, then cut into 1/2 inch thick slices. Serve with mustard sauce.