Combine soy sauce, water, sesame oil, and chili flakes in a small bowl. Set aside.
Heat the oil in a large skillet and cook the garlic and ginger until fragrant, 1 to 2 minutes.
Mix in the whites of the bok choy and cook 3-4 minutes. Add soy sauce mixture and leaves and cook 2 minutes more or until heated through and leaves are gently wilted.
Sprinkle with sesame seeds and serve.
