In a small bowl, whisk together soy sauce, rice vinegar, brown sugar, cornstarch, red pepper flakes (if using), and a pinch of salt and pepper. Set aside.
Heat the vegetable oil in a wok or large skillet over high heat. Add the green beans and stir-fry for 2-3 minutes, until slightly browned and tender-crisp.
Add the garlic and ginger and stir-fry for another 30 seconds, until fragrant.
Pour the soy sauce mixture over the green beans and toss to coat. Continue cooking for 1-2 minutes, until the sauce has thickened and the green beans are coated in a glossy sauce.
Serve hot, garnished with toasted sesame seeds.