Heat a large wok over high heat. Add 2 tablespoons of the oil. When the oil is hot, add the ginger, garlic, and chili flakes and stir-fry just until they are aromatic, about 30 seconds. Scoop out the aromatics and set them aside.
Add the remaining oil to the wok. When it is hot, add the onion pieces and stir-fry until they turn glossy and bright, 1 to 2 minutes.
Add the bok choy and the Chinese broccoli stem pieces. Stir-fry 1 to 2 minutes more.
Add the long beans and the 1-inch scallion pieces. Continue stir-frying until they are bright green and glossy, 1 to 2 minutes more.
Add the Napa cabbage and the Chinese broccoli leaves, along with about 1/3 cup of the hot stock and the reserved aromatics. Continue stir-frying until the vegetables are all tender-crisp, about 2 minutes more. Add the remaining stock, soy sauce, and cornstarch mixture and stir-fry until the vegetables all look lightly glazed with sauce, about 1 minute more.
Transfer the stir-fried vegetables to a heated serving dish. Garnish with the sliced scallions and sesame seeds and serve immediately.