Step 1

Toss together 2 tablespoons soy sauce, Shaoxing wine, and flour in a medium bowl, add steak and toss until coated. Let marinate at room temperature 20 minutes.

Step 2

Meanwhile, heat 1 tablespoon oil in a large wok over medium-high until hot but not smoking. Add onions; cook, stirring often, until translucent and just starting to brown, about 8 minutes. Add remaining 1 tablespoon soy sauce; cook, stirring constantly, until onions absorb soy sauce, about 20 seconds. Transfer to a medium bowl. Carefully wipe wok clean.

Step 3

Heat remaining 1 tablespoon oil in wok over high heat until smoking. Remove beef from marinade and spread in wok in an even layer; discard marinade in bowl. Cook steak, undisturbed, until lightly browned around edges but still pink on top, 2 to 3 minutes. Return onions to wok, and toss to combine. Serve immediately with rice.