Make the sauce: in a saucepan over medium heat, warm the oil.
Add in the onion; cook until the onion begins to brown, 4-5 minutes, stirring occasionally.
Add in the remaining sauce ingredients; simmer 5-10 minutes; remove from the heat.
Let the tenderloins stand at room temperature for 20-30 minutes before grilling.
Trim any excess fat or silver skin from the tenderloins; coat them evenly with the sesame oil.
Season generously with salt and pepper; grill over direct medium heat until well marked on two sides, about 5 minutes, turning once.
Using a brush, cover the tenderloins with a thin coating of the sauce and continue to grill until the pork is barely pink in the center, 10-15 minutes, turning and basting with the remaining sauce every 5 minutes.
Let rest for 3-5 minutes before slicing on the bias; serve warm.