Heat a wok over medium heat. Add oil and heat until shimmering. Add garlic and stir-fry until fragrant, about 30 seconds. Add bok choy and cook for 30 seconds. Add soy sauce and sugar, then season with salt; cook for 1 minute.

Pour water into the wok, cover, and simmer until bok choy leaves are dark green and stems are tender but still firm, 2 to 3 minutes. Remove from the heat.

Drizzle sesame oil over bok choy and season with pepper.