How to Make It
Stir together oyster sauce, garlic, rice wine, sugar, soy seasoning sauce, dark soy sauce, and crushed Kampot peppercorns in a small bowl.
Stir together cornstarch and 2 tablespoons water in a medium bowl until cornstarch is dissolved; add beef, and toss to coat. Add 1/4 cup of the oyster sauce mixture; cover and refrigerate 8 hours or overnight. Cover remaining oyster sauce mixture, and set aside.
Heat oil in a wok over high. Carefully add beef mixture to wok, and cook, shaking wok often, until beef has a brown sear, 4 to 5 minutes. Add red onion and remaining oyster sauce mixture, and cook, shaking wok often, 2 more minutes.
Stir together lime juice, sea salt, and coarsely ground Kampot peppercorns. Place beef, lettuce leaves, and tomatoes on a platter, and serve with lime sauce.