In a large bowl, combine the beef, garlic, brown sugar, 3 tablespoons of soy sauce, oyster sauce, fish sauce, and black pepper. Toss to combine. Let marinate at room temperature for 30 minutes.
2. Step
In small bowl, combine the granulated sugar, remaining 2 tablespoons of soy sauce, the white vinegar, and water. Add the red onion and toss to coat in the dressing. Set aside to soak.
3. Step
On a platter, arrange the watercress and tomato slices.
4. Step
Preheat a large wok or skillet over high heat, then add the canola oil. When the oil is shimmering, swirl around in the pan to coat evenly. Add half of the marinated beef and sear on all sides, shaking the pan, until the meat is slightly charred and medium rare, 3-5 minutes. Remove the meat from the pan and repeat with remaining beef.
5. Step
Arrange the beef on the bed of watercress and tomatoes. Top with the dressed red onions and fresh cilantro.
6. Step
Serve with domed white rice. For a full meal, serve alongside Vietnamese crepes, spring rolls, and bánh mì sandwiches.
