In a small bowl, mix the Hoisin sauce, light soy sauce, rice vinegar, chopped green onions, chopped garlic and chili flakes.
Put the tenderloin in a zip-loc bag or a small container and cover well with the sauce. Put it in the fridge and let the tenderloin marinate for a couple of hours, but preferably overnight.
Preheat the oven to 350 ℉ (180 ℃) with convection (fan).
Take the tenderloin out of the bag or container, transfer it to a baking dish and put it in the oven to bake for 20-25 minutes until it reaches 150℉ (or 65℃) inside temperature. Keep the marinade.
Shortly before the tenderloin is ready, transfer the marinade to a small pot and bring it to a boil. Reduce it to roughly 2/3 the original volume.
When the tenderloin is ready, take it out of the oven and let it rest for 5 minutes.
Cut the tenderloin into 0.8 inch slices and pour the sauce on top. Done!
