Preheat oven to 400 F.
Place bok choy on a rimmed baking sheet. Drizzle with olive oil and a few grinds pepper.
Bake until stalks are tender, 6 to 8 minutes.
Meanwhile, in a small bowl, combine the sesame oil, soy sauce, vinegar, and sugar. Whisk to blend.
Put bok choy on a platter or serving dish, spoon dressing over and sprinkle sesame seeds on top. Serve immediately.