Cut the 1 ½ lb beef sirloin into 1½-inch cubes. Pat every surface very dry with paper towels — this is the single most important step for getting a good sear. In a large mixing bowl, whisk together the 2 tbsp oyster sauce, 1 tbsp soy sauce, 1 tbsp fish sauce, 1 tbsp sugar, 1 tsp sesame oil, ½ tsp black pepper, and 4 cloves garlic minced. Add the 1 ½ lb beef sirloin and toss to coat thoroughly. Cover and marinate in the refrigerator for at least 1 hour, or up to 4 hours. Before cooking, remove the 1 ½ lb beef sirloin from the fridge and let it sit at room temperature for 15–20 minutes.

In a small mixing bowl, combine the 2 tbsp fresh lime juice, ½ tsp salt, and ½ tsp black pepper. Stir until the ½ tsp salt dissolves. Taste and adjust — it should be bright, salty, and peppery. Set aside.

In a medium mixing bowl, whisk together the 1 tbsp olive oil, 1 tbsp rice vinegar, and 1 tbsp sugar. Add the 4 oz watercress and thinly sliced medium red onion very and toss gently to coat. Arrange on a large serving platter. Place the 2 medium tomatoes wedges around the edges. Set aside — the salad will lightly wilt when the hot 1 ½ lb beef sirloin is placed on top, which is intentional.

Remove the 1 ½ lb beef sirloin from the marinade and pat the cubes dry again with paper towels. This second pat-dry is key — excess moisture will steam the 1 ½ lb beef sirloin instead of searing it. Discard the used marinade.

Heat a large wok or heavy skillet over the highest heat your stove allows. Let it get screaming hot — at least 2–3 minutes. Add 1 tablespoon of the neutral oil such as vegetable and swirl to coat. You should see wisps of smoke. Add the 4 cloves garlic smashed cloves and stir for 15 seconds until fragrant but not burnt.

Add half the beef sirloin cubes in a single layer — do not crowd the pan. Cook in two batches if needed. Let them sear undisturbed for 45–60 seconds to build a deep brown crust on the bottom. Then shake the pan vigorously (or use tongs to toss) every 30–45 seconds, searing all sides. Cook for a total of 2–3 minutes for medium-rare, or 3–4 minutes for medium. Transfer to a plate and repeat with the remaining neutral oil such as vegetable and 1 ½ lb beef sirloin.

Return all the beef sirloin to the pan over high heat. Add the 1 tbsp unsalted butter and toss everything together for 30 seconds until the 1 tbsp unsalted butter melts and coats the 1 ½ lb beef sirloin in a glossy, caramelized sauce.

Immediately pile the hot seared 1 ½ lb beef sirloin on top of the prepared 4 oz watercress salad platter. The heat will gently wilt the greens. Serve right away with the 2 tbsp fresh lime juice-pepper dipping sauce on the side. Each bite of beef sirloin gets dipped in the sauce before eating. Serve with steamed jasmine rice.