In a medium bowl, combine 1 tablespoon oyster sauce, 1 tablespoon soy sauce, 2 teaspoons sweet dark soy sauce, 1 teaspoon ground Kampot pepper, 1 teaspoon cornflour (cornstarch), ½ teaspoon baking soda, and 2 finely grated garlic cloves. Mix well until the cornflour and baking soda are fully dissolved and the marinade is smooth and evenly blended.
To make the stir-fry sauce, mix together 2 tablespoons oyster sauce, 1 tablespoon soy sauce, and 1 teaspoon sugar. Stir until the sugar is fully dissolved and the sauce is smooth and well combined. Set aside for later.
In a small bowl, combine 3 tablespoons lime juice, 1 teaspoon ground Kampot pepper, and 1 teaspoon sea salt. Stir well until the salt is mostly dissolved. Set aside until ready to serve.
Heat vegetable oil in a wok or large pan over high heat. Add beef in a single layer and sear for 30 seconds, then toss. Repeat 3 times until lightly charred. Add onions and toss briefly, then pour in the stir-fry sauce. Toss for 30 seconds (or longer for well-done). Turn off heat and let rest in the pan for 1 minute.
Arrange lettuce on a serving plate, top with tomato slices and beef. Serve with steamed rice and dipping sauce.