Cut a thick slice from the pak choi root to separate the leaves. Rinse and drain.

Heat the groundnut oil in a large wok over a medium heat and add 1 tbsp sesame oil, the garlic, chilli, fish sauce (if using) and pak choi. Toss until coated and clamp a pan lid over them. Reduce the heat and cook for 3-6 minutes, tossing occasionally, just until the leaves have wilted (the stalks should be tender-crisp).

Add the rest of the sesame oil and salt. Toss the leaves and serve immediately.