Cook's Note: Spicy sesame oil is available in the Asian section of most supermarkets, but you could also substitute a combination of regular sesame oil and red chilli flakes to taste.
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.