Make sauce: In a medium bowl, whisk the soy sauce with the vinegar, sugar, and cornstarch. Set aside.
In a large skillet over high heat, heat 1 tablespoon oil. Add flank steak, season with salt, and cook until cooked through and seared on all sides, about 8 minutes. Remove steak to a plate.
Add 1 tablespoon remaining oil and red and green bell peppers to the skillet. Cook until softened, about 4 minutes. Add the garlic and ginger and cook stirring until fragrant. Return the beef to the skillet and pour over prepared sauce. Cook, stirring, until the sauce is glossy about 2 minutes more.
Serve with rice.
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