Preheat grill or oven broiler (with rack 3 inches from heat source) on high temperature.
Remove visible fat from tenderloin and discard. Set tenderloin aside.
Combine garlic, ginger, fish sauce, soy sauce, sugar, and sesame oil (optional) in a small dish. Stir marinade until sugar dissolves.
Brush tenderloins with marinade or pour one-third of the marinade evenly over the pork. Place in oven or grill with lid closed.
Every 5 minutes, turn over the tenderloin and add 1 tablespoon of additional marinade, until meat is fully cooked (to a minimum internal temperature of 160 °F).
Let stand for 5 minutes.
Cut 12 slices, each about 1 inch thick. Serve three slices (about 3 oz cooked weight) per serving.