Meanwhile, combine the soy sauce, brown sugar, ginger, sesame oil, ginger, garlic and hot pepper flakes in a self-sealing plastic bag and mix to dissolve the sugar. Add the pork tenderloins and close the bag. Toss the meat in the bag to coat it with the marinade. Let stand at room temperature while the oven heats.
Remove the pork tenderloins from the marinade, and place on the baking sheet. Drizzle about half of the marinade over the pork. Pour the remaining marinade into a small saucepan and add 2 tablespoons water; set aside.
Roast for 15 minutes. Baste the pork with the marinade in the roasting pan. Continue roasting until an instant-read thermometer inserted in the center of the meat reads 150-155°F, about 25 minutes more. The marinade will eventually thicken and scorch, but the foil will protect the pan, so don't worry. Transfer the pork to a cutting board and let stand for 5 minutes.
While the pork is resting, bring the reserved marinade in the saucepan to a boil over high heat. Cut the roast pork crosswise on a slight diagonal into thin slices, and arrange overlapping slices of the pork on a platter. Drizzle the hot marinade over the pork, sprinkle with the scallion, and serve immediately.