in a medium bowl, combine beef, thinly sliced called, ginger, zest, red pepper flakes, cornstarch, and salt, toss to coat. in a small bowl, combine tangerine juice, soy sauce, vinegar, sugar, and 2 tablespoons of water.

heat a are wok or skillet over high heat until hot. add oil and swirl to coat wok. add beef mixture and remaining scallions, stir constantly, until beef is brown. add tangerine juice mixture, cook, stirring, until squce is bubbling and slightly thickened, abut 3 minutes

serve over rice