Cut the leaves into 4 inch strips, cutting thicker stems in half lengthways. Wash well and drain.
Mix together the fish sauce, tamari, sake and water.
Heat a wok over a high heat and when it is smoking add a few spoonfuls of sesame oil and swirl around.
Add a quarter of the greens and toss. Keep on a high heat and fry for a minute then add 1/5 of the sake mix and fry for 30 seconds more. Tip into a large bowl, then cook the rest of the greens in the same way.
Heat up the wok again and add more sesame oil. Tip in the mushrooms and stir over a high heat for 30 seconds. Add the last of the sake mix and cook until it has almost evaporated.
Add the mushrooms to the greens, mix well and serve at once.