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1
Combine oyster sauce, fish sauce, finely chopped garlic, and sugar in a bowl. Add the beef cubes and turn to coat thoroughly. Allow to marinate at room temperature — do not refrigerate. The beef is ready when it has lost its refrigerator chill entirely; cold meat dropped into a hot wok will steam rather than sear.
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2
Mix lime juice, salt, and freshly ground black pepper in a small bowl to form the dipping sauce. Set aside. Spread the watercress across the serving plate.
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3
Heat the wok over the highest available flame until it is visibly smoking — the surface must reach above 300 °C. No oil is added until the wok is at temperature. The wok is ready when a drop of water vaporises on contact instantly and the air above the pan shimmers.
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4
This is where the dish lives or dies. Add a small quantity of oil, then lay the marinated beef cubes in a single layer. Do not move them for 45–60 seconds. A dark, dry crust must form on the base of each cube before any movement begins — premature agitation destroys the sear and releases moisture. Once the crust has formed, shake and toss the wok vigorously to expose all faces of each cube to the hot metal surface. Total cooking time is 2–3 minutes for medium-rare. The beef is done when the exterior is deeply coloured on multiple faces and the cubes yield with slight resistance to a fingertip.
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5
Transfer the beef immediately over the bed of watercress. Serve the lime-salt-pepper dipping sauce alongside. The residual heat will gently wilt the watercress beneath.