Velvet (Optional): If using a budget cut, toss beef cubes with baking soda. Let sit for 45 minutes. Rinse thoroughly under cold water and pat bone-dry with paper towels.
Pickle the Onions: In a small bowl, whisk together the vinegar, warm water, sugar, and salt. Add the sliced red onions. Set aside for at least 20 minutes while you prep the rest.
Marinate: Combine beef with oyster sauce, soy sauce, fish sauce, sugar, pepper, and 1 tbsp oil. Let sit for at least 15 minutes.
Prep the Plate: Arrange a ring of cucumbers and tomatoes around the edge of a platter. Pile the watercress in the center and top it with a heap of the drained pickled onions.
Mix the Dip: In small individual saucers, combine the salt and pepper. Place a lime wedge on the side of each saucer for guests to squeeze in fresh.
The Veggies: Heat your wok to medium-high with a bit of oil. Toss in the red onion chunks and bell peppers. Sauté for 1-2 minutes until they are bright and snappy. Remove and set aside.
The Shake: Add a little more oil to the wok. Add garlic and sear the beef in batches so the pan stays hot. Look for a dark, caramelized crust (about 2 minutes per batch). Return the vegetables back to the wok and toss.
The Finish: Add the butter and the wide-cut scallions. Toss aggressively for 30–60 seconds until the butter creates a glossy glaze over everything.
Serve: Immediately pour the hot beef and all those savory wok juices directly over the pickled onions and watercress. Garnish with long stems of cilantro. Serve with jasmine rice.