1. Stir the soy sauce, sugar, pepper flakes and 2 tablespoons water in a small bowl until the sugar is dissolved. Toss the green beans with the sugar-soy mixture in a large skillet and bring to a boil over medium-high heat. Cover and continue to cook until the beans are crisp-tender, stirring occasionally, 7 to 9 minutes.
2. Uncover and add the ginger and vinegar. Continue to cook, tossing, until the sauce has reduced to a syrupy consistency and is coating the green beans and the beans are tender, 5 to 7 minutes more. Drizzle with the sesame oil and toss well. Serve warm.