Cut the tenderloin into 1-1.5 inch cubes. Lightly season with salt and pepper. Allow it to rest while you prepare the sauce.
In a small bowl, mix all the SAUCE ingredients together until the sugar dissolves. Set aside.
In a condiment dish, combine the DIPPING SAUCE ingredients and set aside.
Heat a wok or skillet over medium-high heat. When the wok gets smoky, coat the wok in oil.
Add sliced onion and stir fry until it's caramelized. Remove from the wok and set aside.
Turn the heat to high and add a bit more oil. Add half of the beef and cook untouched until the meat is nicely browned and releases from the pan easily. Shake or flip each piece over and brown the other side.
Remove the beef and set aside. Repeat with the other half of the batch.
Turn the heat down to medium and add garlic, caramelized onions and the browned beef and toss together.
Add the sauce and turn the heat back to high. Give it a good toss to coat the meat, then leave it alone. Allow it to cook until the sauce begins to reduce and thicken.
Shake the wok to release the tenderloin, give it another toss and allow the sauce to reduce into a glaze.
Turn off the heat and add butter. Shake the wok again to release the beef and give it another toss to coat the meat in butter. Serve Bo Luc Lac with dipping sauce.