Do not preheat oven. Place ham, straight from the refrigerator, on a rack in a shallow roasting pan. Add half cup water. Insert a meat thermometer into the thickest part of the ham. Cover the pan tightly with aluminum foil, leaving the thermometer dial exposed. Roast in a 325 oven until thermometer registers 135 degrees, about 19-23 minutes per pound. Meanwhile, combine rhubarb, sugar, orange juice, orange peel, dry mustard and cinnamon stick in a large saucepan. Bring to a boil. Reduce heat to medium and cook, uncovered, about 15 minutes, stirring occasionally. Remove cinnamon stick. Remove aluminum foil from ham and spoon a small amount of sauce over the ham 15 minutes before end of cooking time. Remove ham when meat thermometer registers 135 degrees. Allow ham to stand, covered, about 10 minutes, or until the thermometer registers 140^. Serve remaining sauce with ham. Makes 2.25 cups sauce.