Bring a pot of water to a boil. Blanch the bok choy for 1½–2 minutes until the stems are just tender but still crisp and the leaves are bright green. Drain immediately and arrange cut-side up on a serving plate so the sauce can pool.
In a small pan, heat the oil over medium heat. Add the minced garlic and chili flakes. Stir for 20–30 seconds until the garlic is fragrant and lightly golden (do not burn it).
Quickly add the soy sauce, oyster sauce (if using), sugar, and water. Let it bubble for 15–20 seconds until slightly thickened and glossy. Turn off the heat and stir in the sesame oil.
Spoon the hot garlic-chili sauce generously over the arranged bok choy so it coats every piece and pools at the bottom of the plate. Sprinkle with sliced green onions.
Serve immediately while the sauce is still sizzling and aromatic, ideally over a mound of fluffy steamed rice so the excess sauce soaks into the grains.
