01 - Combine sliced beef with soy sauce, fish sauce, sugar, oil, minced garlic, and black pepper. Mix thoroughly and marinate for 10 minutes to infuse flavors.
02 - Whisk together fish sauce, lime juice, rice vinegar, sugar, minced garlic, and chili in a small bowl. Stir continuously until sugar completely dissolves.
03 - Heat a large skillet or grill pan over high heat. Sear marinated beef for 2-3 minutes per side until just cooked through. Remove and rest for a few minutes before slicing.
04 - Combine watercress, red onion, cucumber, carrot, cherry tomatoes, cilantro, and mint in a large mixing bowl.
05 - Slice cooked beef into bite-sized pieces. Add to salad, pour dressing over top, and toss gently to coat. Transfer to serving platter and garnish with chopped peanuts. Serve immediately.