Preheat oven to 425 degrees F. In a small bowl combine brown sugar, five-spice powder, and salt; set 1 teaspoon brown sugar mixture aside. Rub remaining brown sugar mixture into pork tenderloin. Place tenderloin in a foil-lined baking pan.
Roast, uncovered, for 20 minutes or until a meat thermometer registers 155 degrees F. Cover with foil and let stand for 5 minutes (meat temperature will rise to 160 degrees F).
Meanwhile, slice the green portion of green onions into thin strips; chop the white portion. In a medium bowl combine green onions, mango, cucumber, jalapeno pepper (if using), and the reserved brown sugar mixture.