In a medium bowl, combine the beef cubes with 2 tablespoons soy sauce, 1 tablespoon oyster sauce, 1 teaspoon sugar, 1/2 teaspoon black pepper, 2 minced garlic cloves (for marinade), and 1 teaspoon sesame oil. Toss to coat evenly and let marinate at room temperature for 10 minutes.

While the beef marinates, prepare the stir-fry sauce: In a small bowl, whisk together 1 tablespoon soy sauce, 1 tablespoon oyster sauce, 1 teaspoon sugar, and 1 tablespoon rice vinegar or lime juice. Set aside.

Heat 1 tablespoon of vegetable oil in a large wok or heavy-bottomed skillet over high heat until shimmering. Add half of the marinated beef in a single layer, ensuring not to overcrowd the pan. Sear for 1-2 minutes per side until browned, then remove the beef from the pan and set aside. Repeat with the remaining beef, adding more oil if needed.

Add the remaining 1 tablespoon of vegetable oil to the same pan. Add the onion wedges and red bell pepper (if using) and stir-fry for 2-3 minutes until slightly softened but still crisp. Add the remaining 2 minced garlic cloves (for stir-fry) and stir-fry for another 30 seconds until fragrant.

Return all the seared beef to the pan with the vegetables. Pour in the prepared stir-fry sauce and toss quickly to coat the beef and vegetables, cooking for 1 minute until the sauce slightly thickens and coats everything.

Arrange the mixed greens, sliced tomato, and sliced cucumber on a serving platter or individual plates. Spoon the Bo Luc Lac over the salad. Serve immediately with cooked jasmine rice and lime wedges on the side. Guests can squeeze lime juice over the beef and mix salt and pepper for a simple dipping sauce.