Place pork in a greased roasting pan. Rub with 1/4 teaspoon each salt and pepper. Combine mustard and next 4 ingredients in a small bowl; spread evenly over pork.
Peel a crosswise stripe around each potato with a vegetable peeler, if desired. Cut each potato in half lengthwise. Toss potatoes with 2 tablespoons oil, 1 tablespoon chopped rosemary, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Add to roasting pan around pork. Insert meat thermometer into thickest part of roast.
Bake at 375° for 1 hour to 1 1/4 hours or until thermometer registers 160°. Let stand 10 minutes. Transfer roast to a serving platter. Surround pork with potatoes. Garnish, if desired.