Cut flank steak lengthwise into strips 1 ½ inches wide; then cut across the grain into 1/8-inch slices. Transfer steak to a mixing bowl. Add marinade ingredients and toss to coat thoroughly. Let stand for 15 minutes or longer in the refrigerator.
Seed peppers and cut into 1-inch squares.
Mix seasoning sauce ingredients in a small bowl.
Add 1 tablespoon oil to a wok over high heat. Swirl the oil and heat for 30 seconds. Add pepper squares and salt, and stir-fry for 1 minute. Transfer to a plate.
Wipe wok with paper towels. Heat 2 tablespoons oil in wok. When hot, add ginger; after a few seconds, add beef. Stir-fry over high heat for 1 to 2 minutes.
Return pepper squares to the wok and pour in sauce. Stir until thickened and serve.