Heat oil in a large nonstick wok, set over medium-high heat. Add the ginger, scallions, and garlic; cook, tossing and stirring, until fragrant, about 30 seconds. Add red-pepper flakes and orange zest, cook 20 seconds longer.
Slip the sirloin strips in the pan and cook, stirring and tossing, until lightly browned, about 2 minutes. Add the broccoli and bell pepper; continue tossing and stirring until slightly softened but still crunchy, about 1 minute.
Pour in the soy sauce and vinegar and bring to a boil, stirring constantly. Add roux and cook, stirring, until thickened, about 15 seconds. Immediately remove from heat. Let stand 1 minute before serving.