Place the bok choy in a steamer basket and place the basket over (not in) simmering water. Steam until the base of the bok choy is just tender when pierced with the tip of a knife, about 6 minutes. Meanwhile, whisk together the chives, soy sauce, lime juice, lime zest and sugar in a small bowl. Transfer the bok choy to a platter and drizzle with the soy mixture. Drizzle with the spicy sesame oil and serve.