For marinade, combine ingredients. Put the pork into a large baggie and pour marinade in. Marinate pork, refrigerated, overnight or as long as you want, turning the bag over occasionally.
For mustard sauce, combine ingredients and refrigerate at least 2 hours.
Grill pork tenderloin 15 to 20 minutes, turning occasionally. Or, if you like, broil it. Cook until most of the pink is gone. If using a thermometer, tenderloin should cook to 160°F–165°F. Slice in thin slices diagonally. Serve with mustard sauce. Makes about 4 to 8 servings. If desired, serve in buns as sandwiches.