Peel and finely slice the onion. Finely slice the mushrooms. Separate the pak choi leaves and finely shred the leaves and stems.
Warm 2 tsp oil in a deep frying pan or wok. Add the chicken. Fry over a medium heat for 10 mins, turning now and then till the chicken is a dark golden brown and cooked through.
While the chicken is cooking, bring a pan of water to the boil. Add 1 nest of noodles. Simmer for 3 mins. Drain and tip back into the pan. Add 1 tsp oil and swirl to mix through the noodles.
Add the onion to the wok with a splash of water. Stir fry for 5 mins till the onion is golden brown. Add the mushrooms. Stir fry for 5 mins till the onion looks juicy.
Lift the chicken out of the pan and shred with two forks. Add the shredded chicken, pak choi, hoisin sauce and 1 tbsp tamari to the wok.
Stir fry for 5 mins to heat everything through and wilt the pak choi.
Add the noodles to the wok. Lift up and drop everything a few times with the fork to mix them all together. Warm for 1-2 mins. Divide the chop suey between two plates and serve straight away.