Place a large pot of water on to boil. Meanwhile, cut the bok choy lengthwise into thirds or quarters and place in a sink full of cold water to soak for several minutes. Wash thoroughly to get any dirt out of the base of the stalks.
When the water is boiling, add about 1 tablespoon salt, bring back to the boil, and add the bok choy. Stir with a long-handled wooden spoon to make sure all the greens are immersed. Bring back to a boil, boil for under a minute, drain, and set aside.
Heat a large wok over high heat. Add the oil and swirl to coat the wok. Toss in the chiles and ginger. Stir briefly, then add the greens and stir-fry for 30 seconds, pressing them against the sides of the wok to sear them a little. Add the broth and let it boil for about 30 seconds. Stir the cornstarch paste well, then add it together with 1/2 teaspoon salt. Stir-fry for another 15 to 30 seconds, turn out onto a small platter, and serve. (Warn your guests that the chiles are not for eating, just for flavor.)
Recipe courtesy Jeffrey Alford and Naomi Duguid