Heat a wok over medium heat. Add oil and allow to get hot. Add garlic and stir-fry until fragrant, about 30 seconds. Add bok choy and cook for 30 seconds. Season with soy sauce, sugar, and salt and stir-fry for 1 minute.
Pour water into the wok, cover, and simmer until leaves are dark green and stems are tender, but still firm, 2 to 3 minutes. Remove from heat, drizzle with sesame oil, and season with pepper.