In a medium bowl, combine the beef, 2 tablespoons soy sauce and ½ teaspoon pepper. Toss to combine and set aside. In a small bowl, stir together 4 tablespoons of the lime juice, the fish sauce, sugar and remaining 1 tablespoon soy sauce.

In a 12-inch skillet over medium-high, heat 1 tablespoon of the oil until beginning to smoke. Swirl to coat the pan, then add the beef in a single layer.

Cook without stirring until well browned, about 1½ minutes. Flip each piece and cook until the second sides are well browned, about another 1½ minutes. Transfer to a medium bowl.

To the same skillet, add the remaining 1 tablespoon oil, the garlic and 1 teaspoon pepper. Cook over low, stirring constantly, until fragrant and the garlic is no longer raw, about 30 seconds.

Pour in the lime juice mixture and any accumulated meat juices, increase to medium-high and cook, stirring constantly, until the liquid is syrupy, and a spoon leaves a trail when dragged across the skillet, 2 to 4 minutes.

Add the beef and cook, stirring and scraping up any browned bits, until the sauce clings lightly to the meat, about 2 minutes. Add the onion and stir until slightly softened, about 1 minute. Remove from the heat.

In a medium bowl, toss the watercress with the remaining 1 tablespoon lime juice and ¼ teaspoon salt. Make a bed of the watercress on a serving platter. Top with the beef mixture and its juices. Serve with lime wedges.