Step 1

Whisk fish sauce, soy sauce, and molasses together in medium bowl. Add beef and toss to coat. Let sit at room temperature for 15 minutes. Spread watercress in shallow serving bowl. Combine lime juice and pepper in small bowl and set aside.

Step 2

Using tongs, transfer beef to second medium bowl, letting as much marinade as possible drain back into first bowl. Add water, garlic, and cornstarch to marinade in first bowl and whisk to combine; set aside. Add 2 teaspoons oil to beef and toss to coat.

Step 3

Heat 1 teaspoon oil in 12-inch nonstick skillet over medium-high heat until just smoking. Using tongs, wipe skillet clean with paper towels. Add half of beef to skillet, leaving space between pieces. Cook, swirling skillet gently and occasionally to capture any fond that collects on bottom of skillet, until beef is browned on first side, 3 to 4 minutes. Continue to cook, stirring and shaking skillet frequently, until beef is coated and browned and center is just pink (to check for doneness, remove larger piece and cut in half), 2 to 4 minutes longer. Transfer beef to clean bowl. Wipe skillet clean with wet paper towels and repeat with remaining 1 teaspoon oil and beef.

Step 4

Melt butter in now-empty skillet over medium heat. Add onion and cook, stirring occasionally, until just beginning to soften, about 1 minute. Add reserved marinade and bring to boil. Cook, stirring occasionally, until thickened and glossy, about 2 minutes. Add beef and any accumulated juices and toss to coat. Scatter beef mixture and sauce over watercress. Drizzle 2 teaspoons lime juice mixture over salad. Divide remaining lime juice mixture among small bowls for dipping and serve with salad.