In a small bowl, combine the oyster sauce, black pepper, 1 tablespoon fish sauce and 1 tablespoon sugar. Place the beef in a large bowl and rub with the mixture. Allow the beef to marinate for a couple minutes.
Add butter or oil to a pan and place the pan over high heat. Cook the meat until the surface begins to caramelize, 4 minutes per side for medium-rare. Continue cooking until the meat reaches the desired doneness. Let the meat rest on a cutting board.
Meanwhile, crush the chiles and garlic with a mortar and pestle. Then, in a large bowl, combine the crushed garlic and chiles, lime juice, shrimp paste, remaining tablespoon sugar and remaining tablespoon fish sauce and mix until well combined. Thinly slice the cooled beef and toss with the dressing. Add the scallions, cucumbers and onions and toss to combine. Add the tomatoes, tossing carefully so they don't smash. Arrange the salad on a large plate, then top with the cilantro and serve with the lettuce either on the side or placed directly under the beef.
Palm sugar is available in most Asian and Hispanic markets. Shrimp paste and oyster sauce are available in Asian markets and most grocery stores.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.