Heat sesame oil in a wok and cook the onion, garlic, ginger and chilli for 5 mins. Then add the mushrooms and cook for 5-10mins until soft and dark brown.
Next, add the chicken and cook for a further 7-10mins.
Meanwhile, in a small bowl mix together the soy sauce, sugar, chilli flakes and peanut butter. Pour this into the wok and mix well, before adding the noodles, let these start to break up before stirring, so as not to break the noodles. Allow the liquid to reduce for 2-4mins.
Give it a good mix before serving, topped with the fresh spring onions.