Bò Luc Lac (Shaking Beef)

From “Vietnamese Home Cooking” p. 196

Ingredients

1-1/2 pounds filet mignon, trimmed of excess fat and cut into 1-inch cubes
1 tablespoon plus 1 teaspoon sugar
1 teaspoon kosher salt
1 teaspoon freshly ground black pepper
1/2 cup plus 1 tablespoon canola oil
1/4 cup rice vinegar
1/4 cup mirin
1/4 cup light soy sauce
1 tablespoon dark soy sauce
2 teaspoons Fish sauce
1 cup thinly sliced red onion
3 whole scallions, trimmed and cut into 1-inch lengths
1 tablespoon finely chopped garlic
2 tablespoons unsalted butter
1 bunch watercress, tough stems removed

Dipping Sauce

2 teaspoons kosher salt
1 teaspoon freshly ground black pepper
4 tablespoons freshly squeezed lime juice

Directions 

1. To marinate the beef, in a bowl, combine the beef, 1 teaspoon of the sugar, salt, pepper, and 1 tablespoon of the oil and stir to mix well. Let marinate at room temperature for 2 hours. 
2. In a bowl, whisk together the vinegar, the remaining tablespoon sugar, mirin, light and dark soy sauces, and fish sauce until the sugar has dissolved. Set aside. 
3. Divide the meat into 2 equal portions. Heat a wok over high heat until hot; the metal will have a matte appearance and a drop or two of water flicked onto its surface should evaporate on contact. Add 1/4 cup of oil and heat until shimmering but not smoking. Add half of the beef in a single layer and sear for about 5 minutes, until a brown crust forms on the first side. Flip the cubes and cook for 1 minute on the second side. 
4. Add half each of the red onion and scallions and cook, stirring, about 30 seconds. Add half of the soy sauce mixture and shake the pan to coat the cubes of beef. Add half each of the garlic and butter and shake the pan to distribute evenly. Transfer to a dish and keep warm. 
5. Wipe the wok clean and return to high heat. Repeat steps 3 and 4 with the remaining 1/4 cup of the oil, beef, red onion, scallions, garlic, and butter. 
6. Arrange the watercress on a platter and top with the beef. To make the dipping sauce, in a small bowl, stir together the salt and pepper and squeeze in the lime juice. Place alongside the platter for dipping the beef cubes as you eat.