Heat the oil in a large sauce pan on a medium-high heat and add the chopped chicken. Cook for 5 minutes, turning regularly until sealed all over.
Add the onion and finely chopped chillies and continue to cook, stirring regularly for 5-6 minutes, until the onion softens.
Add in the ground coriander, cumin, turmeric and peanut butter and stir to coat the chicken.
Stir in the mushrooms, chicken stock and coconut milk. Bring to the boil, then simmer for 20 minutes.
Whilst the soup is simmering, cook the noodles as per the pack instructions (usually this means cook in boiling water for 4 minutes). Drain the noodles and run under cold water to stop the cooking process. Drain again when the noodles are cold.
When the soup has simmered for 20 minutes, add in half the lime juice and a pinch of salt and pepper. Taste, then add more lime juice and seasoning if needed.
Add in the spring onions and allow to cook for a further minutes or two to soften slightly, then stir in the cooked noodles and heat for 1-2 minutes to heat them through.
If you would like the soup to be a little thicker, you can mix the 2tbsp of cornflour with 6 tbsp of cold water, and add a splash at a time to the soup, whilst stirring to thicken.
Turn off the heat and divide the soup between four bowls. I find it easier to use tongs to place the noodles into the bowls first, then ladle the rest of the soup on top.
Sprinkle with the sliced chillies and chilli flakes before serving.