In a dry small fry pan over medium heat, toast the sesame seeds until golden brown and fragrant, 4 to 5 minutes. Transfer to a plate and let cool.
Cut off the tough base from each head of bok choy. Separate the heads into individual stalks by snapping the stalks away from their cores.In a wok or a large fry pan over medium-high heat, warm the canola oil. When it is hot and shimmering in the pan, add the garlic and red pepper flakes and cook, tossing and stirring constantly, until fragrant but not browned, 20 to 30 seconds. Add the bok choy and a pinch of salt and cook, tossing and stirring, until the bok choy just begins to wilt, 1 to 2 minutes. A
dd the broth and cook, stirring occasionally, until the bok choy is just tender and the broth evaporates, 1 to 2 minutes. Add the chili oil, stir well to coat the bok choy and remove from the heat.
Stir in the sesame seeds, transfer the bok choy to a warmed serving bowl and serve immediately. Serves 4.