In a bowl mix together the char siu sauce, scallions, Chinese five spice powder, sesame oil and honey until combined. Marinate the pork tenderloin in this sauce and refrigerate for around 2 hours.
Heat oven to 400 (200C). Place a rack on top of a baking pan and pour the 1/3 cup of water into the pan. When the pork is ready place it on the rack and into the oven.
Cook for 20 minutes then turn the pork and cook for another 20 minutes. Baste every 10 minutes to ensure the tenderloin stays moist.
Rest the pork for 10 minutes. In this time gently toast the sesame seeds on the stovetop in a pan. Sprinkle over the pork, once it has rested slice and serve.