In a mixing bowl, combine the sirloin steak with oyster sauce, soy sauce, sweet dark soy sauce, ground Kampot pepper, cornflour, baking soda, and finely grated garlic. Mix well and refrigerate for at least 30 minutes.

While the beef marinates, prepare the stir-fry and dipping sauces by mixing the ingredients in separate small bowls.

Heat a wok or large frying pan over high heat, add vegetable oil, then add the marinated beef in a single layer and sear for about 30 seconds, tossing until charred and cooked through.

Once the beef is nearly done, quickly toss in the red onion wedges and stir-fry until softened, then pour in the stir-fry sauce, mixing everything together for another 30 seconds.

On a serving platter, layer fresh lettuce leaves at the base, top with sliced tomatoes, then add the beef and onion mixture. Serve with rice and lime dipping sauce.