In a small saucepan, bring the soy sauce, marmalade, honey, rice vinegar, and red pepper flakes to a simmer over medium heat. Remove from the flame and let cool to room temperature. Distribute the glaze evenly in 2 bowls. Marinate the tenderloin roast for a minimum of 1 hour in the glaze.
Heat the grill to medium. Put on the tenderloin. Grill on one side about 6 minutes, depending on size of pork loin. Then flip it, baste it. Grill another 6 minutes. Start checking with a meat thermometer inserted into the thickest part of the meat, at about 6 minutes on the second side. Cook until internal temperature reaches 140 degrees. Remove from heat.
If you have let the temperature get above 140 degrees, you
may need to slice the pork immediately or the meat will continue to cook as it
rests and become overcooked. If you have taken it out in time, let the pork
rest 10 minutes. Thinly slice, sprinkle with the remaining glaze and scallions.