Mix coconut aminos, honey, garlic, ginger, sesame oil, fish sauce, pepper, and chili pepper flakes in a bowl.
Pour into a freezer bag with pork tenderloin and marinate 4 hours or overnight in fridge if possible.
Preheat oven to 425 degrees.
Remove pork from freezer bag and place in baking dish, reserving marinade.
Pour reserved marinade into a small pot over medium heat and bring to a slow boil for 2 or 3 minutes. Gradually whisk in arrowroot starch to thicken.
Once arrowroot is fully incorporated, add onions and allow to cook for 5 or 6 minutes, stirring often.
Spoon thickened glaze over pork tenderloin and place in oven to roast for 30 minutes or until interior temperature reaches 160 degrees, basting twice more with teriyaki sauce during cooking time.
Slice meat and pour additional sauce over pork when serving.
If you're pressed for time, you may skip the marinating and just make the sauce as outlined above. Baste the meat well while cooking and make sure to reserve plenty of teriyaki sauce to pour over meat once sliced. Gluten-free tamari soy sauce can be substituted for coconut aminos.