Instructions

Combine 2 tablespoons soy sauce and 1 teaspoon sugar in medium bowl. Add beef, toss well, and marinate for at least 10 minutes or up to 1 hour, stirring once. Meanwhile, whisk remaining 2 tablespoons soy sauce, remaining 2 tablespoons sugar, broth, mirin, pepper flakes, and cornstarch in medium bowl. Combine garlic, ginger, and 1 teaspoon oil in small bowl.
Drain beef and discard liquid. Heat 1 teaspoon oil in 12-inch nonstick skillet over high heat until just smoking. Add half of beef in single layer, breaking up clumps. Cook, without stirring, for 1 minute, then stir and cook until browned, 1 to 2 minutes. Transfer beef to clean bowl. Heat 1 teaspoon oil in skillet and repeat with remaining beef. Rinse skillet and dry with paper towels.
Add remaining tablespoon oil to now-empty skillet and heat until just smoking. Add mushrooms and cook until beginning to brown, about 2 minutes. Add green beans and cook, stirring frequently, until spotty brown, 3 to 4 minutes. Add water and cover pan; continue to cook until green beans are crisp-tender, 2 to 3 minutes longer. Uncover skillet and push vegetables to sides to clear center; add garlic-ginger mixture to clearing and cook, mashing with spatula, until fragrant, 15 to 20 seconds. Combine garlic-ginger mixture with vegetables. Return beef and any juices to skillet, add scallions, and stir to combine. Whisk sauce to recombine, then add to skillet; cook, stirring constantly, until thickened, about 30 seconds. Serve.