In a large bowl, combine the sirloin cubes with cornflour, baking soda, minced garlic, and a pinch of ground Kampot pepper. Mix thoroughly to coat the beef evenly, then cover and let the mixture marinate for at least 30 minutes at room temperature.

While the beef is marinating, prepare the stir-fry sauce by mixing the oyster sauce, soy sauce, sweet dark soy sauce, sugar, and lime juice in a small bowl. In another bowl, combine lime juice and extra ground pepper for the dipping sauce.

Place a wok over high heat and add vegetable oil. Allow the oil to heat until shimmering.

Add the marinated beef to the hot wok in batches, searing each batch for about 30 seconds on each side until browned. Transfer cooked beef to a plate.

After removing the beef, add more oil if necessary. Toss in red onion and stir-fry for 30 seconds until fragrant. Pour in stir-fry sauce, return beef to wok, and cook for additional 30 seconds.

Turn off the heat and let the Cambodian Pepper Beef rest in the pan for a minute. Serve on lettuce leaves with tomato slices and steamed rice, alongside the tangy dipping sauce.