Cut the stems off the mushrooms and discard. Rinse the mushrooms several times and squeeze out any excess water.
Remove tough stems and any wilted leaves from the choy sum. Rinse at least 3 times and drain.
Heat 2 teaspoons oil in a wok or frypan. Add the mushroom caps and stir fry for a few minutes. Add the soy sauce and stir. Remove from the wok and set aside.
Wipe the wok clean, then add 4 teaspoons oil and heat until very hot. Add the choy sum and stir fry. Add the salt and chicken stock and cook for 2 minutes. Transfer to a plate and arrange the cooked mushrooms on top of the choy sum and serve.