Heat the water in the wok and place the beans in the water. Cover and allow to steam until the beans are crisp tender, about 3 to 4 minutes. Transfer beans to a bowl and cover to keep warm. Pour out the water and dry the wok.
Heat the peanut oil and add the onion slices. Sauté until the onions are soft, about 3 to 4 minutes. Add the garlic and beans, then stir-fry until the beans are hot and just tender, about 1 minute.
Transfer beans and onions to a plate and drizzle on the sesame oil. Season with the sea salt and sprinkle on the sesame seeds, then serve.