Wash the pak choi thoroughly, paying close attention to the base of the stems where grit can collect. Shake off excess water so it doesn’t steam in the pan.
Heat the sesame oil in a wok or wide frying pan over high heat. The pan should be hot before the pak choi goes in.
Lay the pak choi into the pan in a single layer. It should sizzle immediately. Avoid overcrowding. Cook in batches if needed.
Drizzle in the soy sauce and mirin, then sprinkle over the sugar. The heat will quickly coat the stems and leaves with flavor.
Flip the pak choi and scatter over the sesame seeds. Cook briefly until the stems are tender-crisp and the leaves have softened.
Serve hot straight from the pan, with sliced chili on top if you like.
Notes
Use a wide pan and keep the heat high so the pak choi sears instead of steaming.
If the pan looks crowded, cook the pak choi in batches to keep the stems crisp.