Place roast on rack in shallow roasting pan. Brush lightly with oil. In a small bowl, combine mustard, salt and pepper. Rub mixture over roast. Roast pork at 325º for 2 hours or until meat thermometer registers 160º. Remove from oven, cover with foil and let stand 5 to 10 minutes before slicing.
Meanwhile, rinse orange, remove and reserve peel. Section fruit and set aside. Remove white membrane from peel and cut about 2 tbsp julienne strips. In a small sauce pan, combine strips and 1 cup water. Bring to a boil, reduce heat and simmer 15 minutes. Drain and reserve.
In a small sauce pan, combine orange juice and honey. In a small bowl combine cornstarch with 1 tablespoon of water. Stir in orange juice-honey mixture. Cook over medium heat until thickened and bubbly, stirring constantly. Add butter and cooked peel. Continue to cook until butter melts, stirring constantly. Slice roast, garnish with reserved orange sections and sliced mango. Serve with sauce and spinach salad.