Position an oven rack 5 -to -6 inches from the broiler unit, and preheat the broiler. Line a broiler pan or baking sheet with foil, and set the broiler insert on top of the broiler pan if using. Put the pan on the oven rack, and preheat until hot, about 5 minutes.
Stir together 2 tablespoons oil, 1 teaspoon each salt and pepper in a small bowl. Rub the mixture on both sides of the steak. Put the steak on the preheated pan and broil (without flipping) until nicely browned and firm, with some give when pressed in the thickest part, 6 to 8 minutes for medium-rare. Let the steak rest for 10 minutes.
While the steak cooks and rests preheat a wok over high heat for 10 minutes. Have 2 tablespoons of cold water in a small bowl next to the wok. Add the remaining 2 tablespoons oil to the wok, and just as it starts to smoke, add the garlic and ginger. Cook, stirring constantly, for a few seconds until they start to brown, and then immediately add the water. Add the watercress and cook, tossing just until it starts to wilt, about 1 minute. Add the oyster sauce and toss to coat. Remove from the heat and toss for another 30 seconds or so, until the watercress is completely wilted (the stems should be bright green and the leaves darker but not black).
Thinly slice the steak against the grain and serve with the watercress and rice if using.
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