Stir fry for 20 seconds before adding the thinly sliced shiitake or chestnutmushrooms and broccoli florets. Stir fry for 1 minute, then add the pak choi and the sauce mixture. Stir and cook for about 2 minutes over a high heat, adding a splash of water if the sauce becomes too thick.
Tip in the noodles, remove from the heat and toss to mix well. Serve in warmed bowls and tuck in with your chopsticks.