Combine stock, soy sauce, and sherry in a small bowl, and set aside.
Set a wok over high heat for 1 minute. Add oil; swirl to cover bottom and sides. Add string beans; cook, stirring and tossing, 5 minutes. Add garlic, ginger, and reserved stock mixture. Stir to combine; cook 1 minute. Cover; cook until beans are bright green but still crunchy, about 2 minutes. Remove cover; cook until no liquid remains. Sprinkle with sesame seeds, if desired; serve.