In a medium bowl, add the beef cubes, garlic, palm sugar/brown sugar, cornstarch, black pepper, oyster sauce, fish sauce, light soy sauce, and neutral oil. Mix with your hands to evenly coat the beef. Cover with an overturned plate, refrigerate, and marinate overnight or at least 8 hours.
When you’re ready to cook the beef, first prepare your lettuce, cucumbers, tomatoes, and lime. On a serving platter, add a bed of lettuce. Arrange sliced tomatoes and cucumbers on either side of the plate, and add the lime wedges around the perimeter of the plate.
Heat a wok or skillet over medium-high heat until it’s just smoking (or, in the case of a nonstick skillet, just heat it until hot). Add 1 tablespoon of oil, and sear the beef cubes for 5 minutes, tossing periodically to brown all sides. Transfer the beef to a bowl.
Add the onions and another ½ tablespoon of oil. Cook for 2-3 minutes. If the pan is getting dry, add a splash of water to deglaze it.
Add the beef back to the wok, along with the dark soy sauce, oyster sauce, and finally the butter!
Give everything a toss to combine until the sauce is coating the beef and onions. Transfer to your prepared serving platter, on top of your bed of lettuce, and serve with steamed rice.