Preheat the oven to 190°C and roast the marinated meat for 20 minutes. Flip the meat to the other side and roast for another 20 minutes. Brush on the remaining 1 tbsp of Lee Kum Kee Char Siu Sauce , turn off the heat and leave the char siu in the oven to rest for a couple of minutes before slicing.
Marinate the pork loin in Lee Kum Kee Premium Oyster Sauce and 3 tbsp of Lee Kum Kee Char Siu Sauce for at least 30 minutes, ideally overnight. Preheat the oven to 190°C and roast the marinated meat for 20 minutes. Flip the meat to the other side and roast for another 20 minutes.
Pour in half of the char siu sauce (either the homemade version or the Lee Kum Kee Char Siu Sauce) and zip up the back securely. Coat. Knead the sauce into the tenderloin and put in the fridge for at least 4 hours and up to 24 hours.
While the Char siu is cooking, place the reserved half of the sauce in the microwave and cook on high for 45-60 seconds stirring every 15 seconds. You are doing this to thicken the sauce a little. Since it has honey in it, be careful to not overcook otherwise you will have char siu toffee (ask me how I know).