Preparation

• Prepare a medium (350°F to 375°F) gas or charcoal grill fire or heat a grill pan on the stove over medium-high heat. Brush the steak with the oil and season all over with 1/4 tsp. salt. Grill the steak, turning occasionally, until cooked to your liking, about 3 minutes per side for medium rare (125°F). Transfer the steak to a cutting board and let cool slightly, about 10 minutes (if the steak is too hot, it will wilt the greens).
• Meanwhile, in a small bowl, stir together the fish sauce, lime juice, sugar, and jalapeño.
• Thinly slice the steak against the grain. In a large bowl, combine the steak, tomatoes, pineapple, and onion. Add the dressing and toss to combine. Just before serving, put the watercress on a platter, lightly salt, and toss. Top with the steak mixture and peanuts, and serve.