Combine 1/2 teaspoon cornstarch, 1/2 teaspoon sugar, salt, and flank steak in a medium bowl; toss to coat. Set aside.
Combine 2 teaspoons cornstarch, 1/2 teaspoon sugar, broth, soy sauce, and black pepper, stirring with a whisk until sugar dissolves; set aside.
Heat oil in a wok or large nonstick skillet over medium-high heat. Add the green onions, ginger, and garlic; stir-fry 10 seconds. Add beef mixture; stir-fry 3 minutes or until done. Remove the beef mixture from pan; cover and keep warm. Add peas and bell peppers to pan; stir-fry 4 minutes or until crisp-tender. Add beef and broth mixture to pan; cook 2 minutes or until thickened, stirring constantly.