Start by putting a pot with water to boil. This will be used for the noodles.
Over a frying pan, toast the sesame seeds for about a minute or so. Then add the toasted seeds into a mortar and pestle set, and give it a coarse crush.
Into a mixing bowl, place the soy sauce, rice vinegar, sesame oil, chilli oil, spring onions, ginger, and the crushed sesame seeds. Make sure all the mixture is combined well.
Add the noodles into the boiling pot, and cook according to instructions.
About 2 minutes before the noodles are done, roughly chop the Pak Choi and mushrooms and add to the boiling pot.
Once the noodles are ready, removed the water and place it inside a mixing bowl. Add the sauce over the noodles and combine everything well.
Serve the dish hot with an extra drizzle of chilli oil.
