In a container that will accommodate the pork loin, combine the soy sauce, bourbon, and brown sugar. Stir until the sugar is nearly dissolved. Place the pork in dish and turn to evenly coat on both sides. Marinate in the refrigerator for 3 to 4 hours. In a medium bowl, combine the sour cream, mayonnaise, mustard, and vinegar and mix until well blended. Stir in the dry mustard, salt, and green onions. Refrigerate until serving time.
Preheat the oven to 350 degrees F.
Roll the pork loin and bind crosswise at 2-inch intervals with butcher’s twine or kitchen string. Transfer to a baking dish and top with the bourbon marinade. Bake the pork for about 45 minutes, basting with the marinade once during cooking. Let stand about 15 minutes before cutting the twine or string. Carve the pork into thin slices. Pass the creamy mustard sauce to your guests.